Physicochemical Characterisation and Assessment of Oxidative Degradation of Lubricating Oils during Engine Service in Rivers State, Nigeria

Abstract

Lubricating oils undergo progressive chemical and physical changes during engine operation, which can reduce their protective performance. This study assessed the physicochemical characteristics and oxidative degradation of lubricating oil during engine service in Rivers State, Nigeria. Samples were evaluated as fresh oil and after 1,000, 3,000, and 5,000 km of service. Density, kinematic viscosity, viscosity index, total acid number (TAN), total base number (TBN), flash point, moisture content, insoluble matter, peroxide value, p-anisidine value, total oxidation (Totox) value, and oxidative induction time were determined using standard analytical procedures. Data were analysed by one-way ANOVA and Pearson correlation analysis at p < 0.05. Kinematic viscosity at 40°C decreased from 96.42 ± 2.18 to 78.64 ± 2.46 mm²/s, while the viscosity index declined from 152 ± 3 to 131 ± 4. TAN increased from 0.42 ± 0.04 to 2.48 ± 0.16 mg KOH/g, whereas TBN decreased from 9.84 ± 0.28 to 4.26 ± 0.24 mg KOH/g. Peroxide value and Totox value increased to 11.64 ± 0.72 meq O₂/kg and 38.14, respectively, while oxidative induction time decreased to 8.26 ± 0.48 h at 5,000 km. Service distance showed strong associations with the principal deterioration indicators. These findings indicate progressive lubricant degradation with increasing service distance and support condition-based monitoring when establishing oil-replacement intervals.