The Impact of Operating Mileage on the Properties of Libyan and Imported 10W-40 Engine Oils

Abstract

This study explores how the physical and chemical properties of various engine oils change when subjected to real driving conditions over different mileage intervals. Four commercial 10W-40 engine oils were examined, THURAYA, ALMONG, and MD (all locally produced in Libya), along with RAVENOL (an imported brand), to ensure consistency in comparison. Oil samples were collected after operating distances of 1,000 km, 2,500 km, and 4,000 km, with the vehicle maintained at a constant speed of 130 km/h. Fresh, unused oil samples were also analyzed and served as reference points. The evaluation focused on key performance indicators such as viscosity at 40 °C and 100 °C, flash point, and the concentration of essential metals including calcium, zinc, and phosphorus, measured using standard laboratory techniques. The findings revealed a clear deterioration in oil quality as mileage increased. Visually, the used oils showed noticeable darkening and loss of the transparent, glossy appearance defined by Libyan standards. Viscosity values declined steadily with distance, with THURAYA oil maintaining the best stability, while ALMONG oil exhibited the greatest reduction at both test temperatures. Elemental analysis showed variations in calcium, zinc, and phosphorus levels, indicating differences in additive depletion, evaporation of lighter oil fractions, and engine wear. Among the tested samples, RAVENOL oil contained the highest calcium concentration, suggesting superior anti-wear and detergent properties, whereas ALMONG oil recorded comparatively lower calcium content. The flash point values declined progressively with continued operation, ultimately falling outside the allowable limits defined by the Libyan standard, which indicates contamination and deterioration of the oil constituents. In general, the findings revealed that operating mileage has a noticeable effect on engine oil performance and highlight the necessity of routine oil condition monitoring, and timely replacement to preserve the engine efficiency, minimize wear, and prolong its operational lifespan.