The research focuses on the relationship between automotive mileage and its effect on engine oil deterioration. The effects of the new (2020) and old (2012) engines on the most important parameters, kinematic viscosity, flash point, and fire point were investigated. Measurements were performed on four-engine oil samples. Profi-car Endurance SAE 10W-30 semi-synthetic was used for all the engines. The maximum trip length for the oil samples was 8,000 Km. Kinematic viscosity was measured by using a KV1000 Bath (according to the ASTM D445). In addition, the flash point and fire point of the samples were determined by using the Open Cleveland method. This work investigates the difference between old and new engines in actual use on the roads in the rate of decrease in engine oil properties. The cumulative results obtained help drivers protect their car engines by changing engine oil when the engine’s mileage is above 100,000 Km. The comparison of the average results shows that the rate of deterioration of used oil in old engines is higher than in the new engines. It has been shown that the rate of difference in oil viscosity deterioration between old and new engines at cold temperatures, 40°C, and 100°C were 9.3%, 10%, and 4.78% respectively. Also, the different rate for the oil flash points and fire points was 5.56% and 6.75%.