Extended Drain Intervals Without Compromising Engine Protection? A Field Evaluation of SAE 15W40 CI-4 Diesel Engine Oil Under Mixed Terrain Operations

Heavy-duty diesel engines operating in commercial transportation fleets are exposed to continuously changing mechanical loads, temperatures, and driving conditions. These demanding environments accelerate lubricant degradation and challenge conventional engine oil replacement schedules. While manufacturers typically recommend conservative oil drain intervals, advances in lubricant formulation and condition monitoring have opened opportunities for extending service intervals without sacrificing engine protection. The study reviewed in this article investigates whether SAE 15W40 API CI-4 mineral diesel engine oil can safely remain in service for up to 20,000 km under real operating conditions involving both inclined and plain terrain. Through standardized ASTM laboratory testing and field-based monitoring of commercial passenger buses, the research provides valuable evidence supporting condition-based maintenance strategies for modern diesel fleets.

Bibliographic Information

Item	Information
Article Title	Extended drain interval performance of SAE 15W40 CI-4 diesel engine oil under plain and inclined terrain operations
Author	Kim Michael Angelo R. Miraflores
Affiliation	College of Engineering, Xavier University – Ateneo de Cagayan, Philippines
Journal	Innovation in Engineering
Publisher	Researcher and Lecturer Society
Volume	3
Issue	1
Publication Year	2026
Pages	69–78
DOI	https://doi.org/10.58712/ie.v3i1.45
ISSN	3047-5473
License	Creative Commons Attribution 4.0 International (CC BY 4.0)
Article Type	Research Article
Received	27 December 2025
Revised	16 March 2026
Accepted	22 March 2026
Keywords	diesel engine oil; extended drain interval; oil condition monitoring; viscosity; TBN

Research Background

Engine lubricants are fundamental to the reliability, durability, and efficiency of heavy-duty diesel engines. Besides minimizing friction between moving components, lubricating oil dissipates heat, prevents corrosion, suspends contaminants, neutralizes acidic combustion by-products, and protects engine surfaces from premature wear. As commercial transportation fleets operate under increasingly demanding conditions, maintaining lubricant performance becomes essential for ensuring engine longevity and reducing operational costs.

Heavy-duty passenger buses often experience fluctuating operating conditions resulting from variations in terrain, traffic density, payload, and driving behavior. Routes involving prolonged uphill travel impose greater brake mean effective pressure (BMEP), elevated lubricant temperatures, and higher mechanical loading than operations on relatively flat roads. These conditions can accelerate oxidation, additive depletion, viscosity changes, and other degradation mechanisms that ultimately determine when engine oil should be replaced.

Most original equipment manufacturers (OEMs) specify fixed drain intervals intended to provide a conservative safety margin for a wide range of operating environments. However, these generalized recommendations may not fully reflect actual lubricant condition during service. Advances in lubricant chemistry, additive technology, and Oil Condition Monitoring (OCM) have encouraged the adoption of condition-based maintenance strategies that replace engine oil according to measured degradation rather than predetermined mileage alone.

Used Oil Analysis (UOA) has become one of the most reliable approaches for evaluating lubricant health during engine operation. Parameters such as kinematic viscosity and Total Base Number (TBN) provide quantitative indicators of lubricant stability, additive retention, and the ability to continue protecting engine components. These standardized measurements enable maintenance engineers to assess whether engine oil remains suitable for continued service beyond conventional replacement intervals.

Despite extensive laboratory investigations into lubricant degradation, relatively few studies have evaluated extended drain intervals under real commercial operating conditions. Laboratory simulations can isolate specific degradation mechanisms, but they cannot completely reproduce the combined effects of varying engine loads, terrain profiles, combustion characteristics, environmental conditions, and operational variability encountered by public transportation fleets. Consequently, field-based evidence supporting extended drain intervals remains limited.

The reviewed study addresses this practical knowledge gap by investigating the performance of SAE 15W40 API CI-4 mineral multigrade diesel engine oil during a 20,000 km extended drain interval under actual fleet operations. Four commercial passenger buses equipped with different heavy-duty diesel engines operated across routes consisting of inclined, plain, and mixed terrain without oil replacement or filter changes throughout the monitoring period. Oil condition was periodically evaluated using internationally recognized ASTM testing procedures, allowing the researchers to assess lubricant degradation under authentic service conditions rather than controlled laboratory environments.

By combining field experimentation with standardized physicochemical analysis and statistical evaluation, the research contributes practical evidence regarding the feasibility of extending drain intervals while maintaining lubricant functionality. The findings are particularly relevant for fleet operators seeking to optimize maintenance schedules, reduce operating costs, and implement condition-based maintenance programs without compromising engine reliability.

Research Objective

The primary objective of this study was to evaluate the performance of SAE 15W40 API CI-4 mineral multigrade diesel engine oil during an extended drain interval of 20,000 km under real-world commercial fleet operations involving both inclined and plain terrain.

To achieve this objective, the research pursued several specific goals:

Assess the stability of engine oil viscosity at 40°C and 100°C throughout an extended service interval using ASTM D445 standardized testing.
Evaluate Total Base Number (TBN) depletion using ASTM D2896 to determine whether sufficient alkalinity reserve remained available for neutralizing acidic combustion products during prolonged operation.
Compare lubricant degradation behavior between buses operating under inclined terrain and those operating primarily on plain terrain to determine whether terrain significantly influences oil performance.
Apply regression analysis and statistical testing to quantify degradation trends and determine whether differences between operating conditions are statistically significant.
Provide field-based evidence supporting or challenging the feasibility of implementing extended oil drain intervals within condition-based maintenance programs for heavy-duty diesel transportation fleets.

Why This Research Matters

The significance of this research extends beyond lubricant testing because it addresses one of the most practical maintenance questions faced by fleet operators: Can engine oil remain in service longer without compromising engine protection? Answering this question has direct implications for maintenance cost, vehicle availability, operational efficiency, and environmental sustainability.

Supports condition-based maintenance. Rather than relying exclusively on fixed manufacturer drain intervals, the study demonstrates how standardized oil condition monitoring can provide objective evidence for maintenance decision-making.
Reduces operating costs. Safely extending oil replacement intervals has the potential to lower lubricant consumption, maintenance labor, vehicle downtime, and associated servicing expenses for commercial transportation fleets.
Provides valuable field evidence. Unlike many previous investigations conducted under laboratory conditions, this research evaluates lubricant performance during actual commercial bus operations, making the findings directly relevant to fleet management practice.
Improves maintenance reliability. Continuous monitoring of viscosity and TBN enables maintenance engineers to detect lubricant degradation before critical failure occurs, reducing the likelihood of premature engine wear.
Enhances fleet sustainability. Extending drain intervals without compromising lubricant integrity decreases waste oil generation and reduces the environmental footprint associated with routine maintenance activities.
Strengthens evidence-based maintenance policies. The research provides scientifically validated data that transportation companies may use when developing maintenance schedules tailored to actual operating conditions instead of generalized mileage recommendations.
Demonstrates the influence of terrain. By comparing inclined and plain operating environments, the study offers practical insights into how route characteristics affect lubricant aging, supporting more customized maintenance planning for mixed-duty transportation fleets.

Research Methodology

This study employed a longitudinal field-based experimental design to investigate the performance of SAE 15W40 API CI-4 mineral multigrade diesel engine oil during an extended drain interval of up to 20,000 km. Rather than relying on laboratory simulations, the researchers conducted the investigation under actual commercial operating conditions to capture the combined effects of engine loading, terrain, temperature variation, and real driving behavior on lubricant degradation.

The experimental design focused on determining whether engine oil could continue providing adequate lubrication and chemical protection beyond the manufacturer's conventional replacement interval while operating across different terrain profiles.

Field Test Vehicles

Four passenger buses operating in commercial transportation service were selected as the experimental fleet. The vehicles represented three commonly used heavy-duty diesel engine manufacturers:

Yuchai YC6L (234 kW)
Weichai WP7 (199 kW)
Cummins ISDe140 (103 kW)
Weichai WP6220E50 (158 kW)

These buses operated under different duty cycles consisting of inclined, plain, and mixed-terrain routes. Their varying operating conditions enabled comparison of lubricant degradation under realistic mechanical loading rather than under controlled laboratory environments.

Lubricant Selection

To eliminate variability caused by different oil formulations, every test vehicle used the same batch of SAE 15W40 API CI-4 mineral multigrade diesel engine oil. Selecting a single lubricant formulation ensured that any observed physicochemical changes originated from operating conditions rather than manufacturing differences.

The fresh oil baseline was characterized before testing using standardized ASTM procedures. Reference properties included:

Kinematic viscosity at 40°C
Kinematic viscosity at 100°C
Viscosity Index
Flash Point
Total Base Number (TBN)
Density

These baseline measurements served as the reference values against which all subsequent oil samples were evaluated throughout the 20,000 km monitoring period.

Sampling Strategy

Oil samples were collected at predetermined mileage intervals:

Fresh oil (0 km)
5,000 km
10,000 km
15,000 km
20,000 km

Sampling was performed immediately after engine operation while the lubricant remained at operating temperature. This procedure minimized particle settling and ensured representative oil samples. Vacuum extraction through the dipstick tube, sterile containers, and controlled handling procedures were employed to prevent contamination during sampling.

Importantly, no oil replacement, oil top-up, flushing, or filter replacement was performed during the entire 20,000 km evaluation period. This conservative testing strategy allowed lubricant degradation to be observed under uninterrupted service conditions.

Laboratory Analysis

Oil condition was evaluated using internationally recognized ASTM analytical methods widely adopted in used oil analysis (UOA):

Parameter	ASTM Standard	Purpose
Kinematic Viscosity (40°C)	ASTM D445	Evaluate lubricant flow characteristics and detect thickening or thinning.
Kinematic Viscosity (100°C)	ASTM D445	Assess viscosity under engine operating temperature.
Total Base Number (TBN)	ASTM D2896	Measure remaining alkalinity reserve available to neutralize acidic combustion products.

The researchers adopted widely accepted acceptance criteria for lubricant serviceability. Viscosity changes were evaluated relative to fresh oil values, while TBN was considered acceptable provided that it remained above approximately 50% of the fresh oil baseline.

Statistical Analysis

Lubricant degradation was quantified using linear regression models that related each oil property to accumulated mileage. Analysis of Variance (ANOVA) was subsequently applied to determine whether degradation trends differed significantly between buses operating under inclined terrain and those operating on predominantly plain terrain. Statistical significance was evaluated at a confidence level of α = 0.05.

This analytical approach enabled the researchers not only to observe lubricant degradation but also to determine whether terrain produced statistically meaningful effects on oil performance.

Key Findings

1. Engine Oil Maintained Stable Viscosity Throughout 20,000 km

One of the most important findings was the remarkable stability of lubricant viscosity during the extended drain interval. Measurements performed at both 40°C and 100°C remained within acceptable operating limits throughout the entire 20,000 km service period.

At engine operating temperature (100°C), viscosity consistently remained within the SAE 40 classification. The results indicate that the lubricant maintained sufficient film strength to protect engine components without exhibiting excessive thickening due to oxidation or excessive thinning caused by fuel dilution or mechanical shear.

This stability demonstrates that the lubricant formulation retained its rheological properties even after prolonged commercial operation under varying terrain conditions.

2. Total Base Number (TBN) Remained Above Critical Limits

The second major finding concerns the lubricant's chemical stability. Although Total Base Number gradually decreased throughout engine operation, every oil sample remained comfortably above the commonly accepted condemnation limit of approximately 50% of the fresh oil value.

The gradual decline in TBN reflected the expected consumption of detergent and dispersant additives responsible for neutralizing acidic combustion products. However, depletion occurred slowly and consistently, indicating that sufficient alkalinity reserve remained available to continue protecting internal engine components.

3. Terrain Had Only a Limited Influence on Lubricant Performance

Buses operating on inclined routes experienced greater engine loading and higher thermal stress than those operating primarily on flat terrain. As expected, lubricant degradation tended to occur slightly faster under these more demanding conditions.

Nevertheless, statistical analysis demonstrated that differences between inclined and plain terrain operations were not statistically significant (p > 0.05). This finding indicates that terrain influenced degradation rate only moderately and did not substantially reduce lubricant serviceability during the evaluated interval.

4. Lubricant Performance Remained Within Acceptable Service Limits

Neither viscosity measurements nor TBN values reached unacceptable levels throughout the study. No evidence of abnormal oxidation, severe additive depletion, excessive viscosity increase, viscosity loss, or accelerated lubricant failure was observed in any of the monitored vehicles.

These observations demonstrate that the tested SAE 15W40 API CI-4 engine oil maintained both its physical and chemical protective functions throughout the entire monitoring period.

5. Extended Drain Intervals Are Feasible Under Condition Monitoring

The central conclusion emerging from the investigation is that extending the oil drain interval to 20,000 km is technically feasible when lubricant condition is continuously monitored using standardized oil analysis techniques.

Rather than relying exclusively on predetermined mileage schedules, maintenance decisions can be guided by measurable lubricant condition indicators such as viscosity and TBN. This condition-based approach enables fleet operators to optimize maintenance schedules while maintaining engine reliability.

6. Field Validation Strengthens Practical Applicability

Unlike laboratory-based degradation studies, this investigation evaluated lubricant performance during actual commercial passenger bus operations. Consequently, the findings more accurately represent the complex combination of thermal loading, mechanical stress, combustion conditions, route characteristics, and operational variability encountered in real transportation fleets.

This practical validation enhances the relevance of the research for maintenance engineers, fleet managers, lubricant specialists, and transportation companies seeking scientifically supported strategies for extending engine oil service intervals.

Scientific Contribution

Although engine oil degradation has been widely investigated, this study makes several meaningful contributions by emphasizing real-world field validation rather than controlled laboratory experimentation. The work demonstrates how standardized oil condition monitoring can support evidence-based maintenance decisions for heavy-duty diesel fleets operating under varying terrain conditions.

Provides rare field-based evidence on extended drain interval performance using commercial passenger buses operating under authentic service conditions.
Demonstrates the effectiveness of ASTM-standardized oil condition monitoring for evaluating lubricant health through viscosity and Total Base Number measurements.
Quantifies the influence of terrain by comparing lubricant degradation under inclined and plain operating environments using statistical analysis rather than qualitative observation.
Supports the transition from fixed maintenance schedules to condition-based maintenance (CBM), providing practical evidence that lubricant replacement decisions can be guided by measured oil condition rather than mileage alone.
Strengthens engineering knowledge regarding lubricant durability by demonstrating that SAE 15W40 API CI-4 mineral diesel engine oil can maintain functional integrity throughout an extended 20,000 km service interval under monitored operating conditions.

Industrial Implications

The findings have direct relevance for transportation companies, fleet maintenance organizations, engine manufacturers, lubricant suppliers, and maintenance engineers responsible for heavy-duty diesel fleets.

Lower maintenance costs through safely extending oil replacement intervals when supported by routine oil analysis.
Reduced vehicle downtime, allowing buses and commercial vehicles to remain in operation longer between maintenance events.
Improved fleet availability, increasing operational efficiency without sacrificing engine protection.
Reduced lubricant consumption and waste oil generation, contributing to more environmentally sustainable fleet management.
Encourages wider implementation of Oil Condition Monitoring (OCM) as a routine predictive maintenance tool in commercial transportation fleets.
Supports maintenance optimization by replacing conservative mileage-based servicing with evidence-based decision making supported by laboratory measurements.
Provides confidence for fleet operators considering extended drain intervals under mixed operating conditions while maintaining engine reliability and component durability.

Research Limitations

Like any field-based engineering investigation, this study has several limitations that should be considered when interpreting its findings. Although the results provide strong evidence supporting extended drain intervals under monitored operating conditions, the conclusions are bounded by the experimental scope, vehicle selection, lubricant formulation, and evaluation parameters used throughout the investigation.

Limited number of test vehicles. The investigation involved only four commercial passenger buses equipped with Yuchai, Weichai, and Cummins diesel engines. Although these engines represent common heavy-duty applications, a larger sample size would provide greater statistical confidence and improve generalizability across different fleet compositions.
Single lubricant formulation. The study evaluated only one SAE 15W40 API CI-4 mineral multigrade diesel engine oil. Consequently, the conclusions should not automatically be extended to other lubricant grades, synthetic oils, semi-synthetic formulations, or oils meeting different API performance classifications.
Maximum evaluation distance of 20,000 km. The research demonstrates lubricant performance only up to the monitored service interval. The study does not determine whether acceptable lubricant performance can be maintained beyond 20,000 km.
Limited physicochemical parameters. Lubricant condition was primarily assessed through kinematic viscosity and Total Base Number (TBN). Other diagnostic indicators such as oxidation, nitration, sulfation, soot concentration, fuel dilution, water contamination, particle count, ferrography, or elemental wear metal analysis were outside the scope of the investigation.
Specific operational environment. The buses operated under commercial passenger transportation routes consisting of inclined, plain, and mixed terrain. Different operating environments—including mining equipment, construction machinery, agricultural vehicles, or long-haul freight transport—may produce different lubricant degradation characteristics.
No direct engine wear assessment. The investigation focused on lubricant condition rather than direct mechanical inspection of engine components. Therefore, although lubricant properties remained within acceptable limits, long-term wear characteristics of internal engine components were not directly evaluated.
Environmental variability. Factors such as ambient temperature, fuel quality, driving behavior, traffic congestion, and maintenance practices may influence lubricant degradation and were not independently isolated during the field experiment.

Future Research Opportunities

The study establishes a valuable foundation for future investigations into extended lubricant service intervals under real operating conditions. Several opportunities exist to expand the current research and strengthen the scientific understanding of lubricant degradation in commercial diesel engines.

Evaluate lubricant performance beyond 20,000 km to determine the practical upper limit of safe drain intervals under continuous oil condition monitoring.
Compare mineral, semi-synthetic, and fully synthetic diesel engine oils operating under identical commercial fleet conditions.
Expand the study to include larger transportation fleets operating across different countries, climates, and duty cycles.
Incorporate comprehensive used oil analysis (UOA) parameters, including oxidation, nitration, sulfation, soot loading, fuel dilution, water contamination, wear metals, particle counting, and ferrographic analysis.
Develop predictive degradation models using machine learning and artificial intelligence to estimate remaining useful lubricant life based on continuous monitoring data.
Integrate Internet of Things (IoT) sensors for real-time lubricant condition monitoring and predictive maintenance in commercial transportation fleets.
Investigate the influence of biodiesel blends, alternative fuels, and renewable diesel on lubricant degradation during extended drain intervals.
Conduct economic analyses comparing conventional fixed drain intervals with condition-based maintenance strategies to quantify lifecycle cost savings.
Evaluate relationships between lubricant degradation and long-term engine durability through teardown inspections and wear analysis.
Develop intelligent fleet maintenance decision-support systems that integrate lubricant monitoring, vehicle telematics, operating conditions, and predictive analytics into automated maintenance scheduling.

Potential for Public Policy Citation (Overton)

This article demonstrates meaningful potential for citation in transportation, environmental, and industrial maintenance policy documents because it provides field-based evidence supporting condition-based maintenance rather than strictly mileage-based servicing. The research contributes to broader discussions surrounding maintenance optimization, resource efficiency, and sustainable fleet management.

Government transportation agencies responsible for public transit systems may find the study particularly valuable when developing technical guidelines for fleet maintenance. The findings suggest that standardized lubricant monitoring can support evidence-based maintenance scheduling, potentially reducing maintenance costs while maintaining operational reliability.

The research is also relevant to environmental policy because safely extending lubricant drain intervals may reduce waste oil generation, decrease lubricant consumption, and lower the environmental footprint associated with routine vehicle servicing. These outcomes align with broader sustainability objectives emphasizing efficient resource utilization and circular maintenance practices.

While the study is based on one lubricant formulation and a relatively small commercial fleet, its methodological approach offers a practical framework that transportation authorities, public fleet operators, and maintenance regulators may consider when developing future technical recommendations related to predictive maintenance and oil condition monitoring.

Who Should Read This Paper?

Mechanical engineers specializing in internal combustion engines.
Tribology and lubrication researchers.
Fleet maintenance engineers and maintenance planners.
Commercial transportation companies operating heavy-duty diesel vehicles.
Automotive service managers implementing predictive maintenance programs.
Researchers working in Oil Condition Monitoring (OCM) and Used Oil Analysis (UOA).
Lubricant manufacturers developing next-generation heavy-duty engine oils.
Graduate students in mechanical, automotive, and transportation engineering.
Government agencies responsible for public transportation fleet management.
Engine manufacturers interested in optimizing maintenance recommendations.
Professionals involved in sustainable transportation and lifecycle asset management.

Final Thoughts

This study presents a carefully designed field investigation into one of the most practical questions facing modern fleet maintenance: can diesel engine oil safely remain in service beyond conventional replacement intervals? By evaluating SAE 15W40 API CI-4 mineral diesel engine oil under actual commercial operating conditions rather than laboratory simulations, the research provides evidence that directly reflects the realities of public transportation fleets.

A major strength of the investigation lies in its emphasis on standardized oil condition monitoring using internationally recognized ASTM analytical methods. The combination of kinematic viscosity measurements, Total Base Number evaluation, regression analysis, and statistical comparison provides a transparent and scientifically robust basis for assessing lubricant health throughout an extended 20,000 km service interval.

The findings demonstrate that the tested lubricant maintained acceptable physicochemical performance throughout the monitoring period, with viscosity remaining within service limits and sufficient alkalinity reserve retained to continue protecting engine components. Although buses operating on inclined terrain experienced slightly greater degradation trends, statistical analysis indicated that terrain did not significantly affect lubricant serviceability within the evaluated interval.

Perhaps the study's most important contribution is its support for condition-based maintenance. Rather than advocating universally longer drain intervals, the research shows that maintenance decisions should be guided by objective measurements of lubricant condition. This evidence-based approach enables fleet operators to optimize maintenance schedules while maintaining engine reliability, reducing unnecessary servicing, lowering operating costs, and minimizing environmental impacts associated with waste lubricant generation.

Overall, this article represents a valuable contribution to diesel engine lubrication research, predictive maintenance, and transportation engineering. Its combination of field validation, standardized laboratory analysis, and practical engineering relevance makes it a useful reference for researchers, fleet operators, lubricant manufacturers, and maintenance professionals seeking scientifically supported strategies for improving diesel engine maintenance practices.

Suggested Citations

UNP–Teknomekanik Style

Miraflores, K. M. A. R. (2026). Extended drain interval performance of SAE 15W40 CI-4 diesel engine oil under plain and inclined terrain operations. Innovation in Engineering, 3(1), 69–78. https://doi.org/10.58712/ie.v3i1.45

APA (7th Edition)

Miraflores, K. M. A. R. (2026). Extended drain interval performance of SAE 15W40 CI-4 diesel engine oil under plain and inclined terrain operations. Innovation in Engineering, 3(1), 69–78. https://doi.org/10.58712/ie.v3i1.45

IEEE Style

K. M. A. R. Miraflores, "Extended drain interval performance of SAE 15W40 CI-4 diesel engine oil under plain and inclined terrain operations," Innovation in Engineering, vol. 3, no. 1, pp. 69–78, 2026, doi: 10.58712/ie.v3i1.45.

Harvard Style

Miraflores, K.M.A.R., 2026. Extended drain interval performance of SAE 15W40 CI-4 diesel engine oil under plain and inclined terrain operations. Innovation in Engineering, 3(1), pp.69–78. Available at: https://doi.org/10.58712/ie.v3i1.45.

Vancouver Style

Miraflores KMAR. Extended drain interval performance of SAE 15W40 CI-4 diesel engine oil under plain and inclined terrain operations. Innovation in Engineering. 2026;3(1):69–78. Available from: https://doi.org/10.58712/ie.v3i1.45

Chicago (Author–Date)

Miraflores, Kim Michael Angelo R. 2026. "Extended Drain Interval Performance of SAE 15W40 CI-4 Diesel Engine Oil under Plain and Inclined Terrain Operations." Innovation in Engineering 3 (1): 69–78. https://doi.org/10.58712/ie.v3i1.45.

MLA (9th Edition)

Miraflores, Kim Michael Angelo R. "Extended Drain Interval Performance of SAE 15W40 CI-4 Diesel Engine Oil under Plain and Inclined Terrain Operations." Innovation in Engineering, vol. 3, no. 1, 2026, pp. 69–78. Researcher and Lecturer Society, https://doi.org/10.58712/ie.v3i1.45.

Editorial Note

This review has been prepared independently for the Engineering Research Insights blog using the published research article as the sole source of scientific analysis. Bibliographic metadata, including the article title, author, journal information, publication details, DOI, publisher, license, and keywords, have been verified against the official article webpage published by the journal. Scientific interpretations, methodology summaries, research findings, and critical discussions are based exclusively on the contents of the published article. This review is intended for educational and scholarly communication purposes and does not replace the original publication. Readers are encouraged to consult the original article for complete experimental procedures, datasets, and technical discussions.

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An in-depth review of the study evaluating SAE 15W40 API CI-4 diesel engine oil under a 20,000 km extended drain interval using ASTM oil condition monitoring. Discover the methodology, key findings, engineering contributions, industrial implications, and the future of condition-based maintenance for heavy-duty diesel fleets.

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Article Summary

This study provides compelling field-based evidence that SAE 15W40 API CI-4 mineral diesel engine oil can maintain acceptable physicochemical performance throughout a monitored 20,000 km service interval under mixed terrain operations. Through standardized ASTM viscosity and Total Base Number analyses conducted on commercial passenger buses operating under real transportation conditions, the research demonstrates that lubricant degradation remained within acceptable limits despite varying engine loads and terrain profiles. Rather than promoting universally longer drain intervals, the study highlights the importance of condition-based maintenance supported by routine oil analysis. Its practical engineering relevance, rigorous experimental methodology, and direct applicability to commercial fleet operations make this article an important contribution to tribology, predictive maintenance, and transportation engineering.

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