Abstract The lubricating oil of an internal combustion engine (i.c.e.) performs several functions having a major contribution to the reliability of the system, implying proper operation during the period of exploitation. Replacing the engine oil ahead reaching the turnover / time corresponding to preventive maintenance it turns out to be sometime an imperative action, with negative implications on operating costs. Any user of today vehicles desire to use the engine performances at full percentage, and for that an important condition becomes the changing of the oil at defined intervals of time/rollover intervals. Thus, it is very important to be aware when to replace because a late change of oil can affect the engine operation and, surely, the performance of the vehicle. Premature oil change influences costs, the environment and is considered a harmful action in terms of the process of depletion of resources. As regards the operating conditions, it is intended that they are similar for all vehicles taken into consideration. It is imperative that the study be carried out by investigating the operation of a single type of engine, with a single type of engine oil, for several vehicles with the same technical characteristics. It is considered that the transport fleets are the concrete source that falls under the already mentioned conditions and a methodology based on the analysis of the 10W30 oil samples used in the MX - 11 330 H2, EURO 6 engines of the DAF vehicles within the fleet of one transport company from Argeş county will be used. The authors of this paper will analyze the following properties of the oil: viscosity, density, flash point, contamination with fuel and water content, these being the most important in illustrating the degree of wear and tear of it. The information about the evolution during the exploitation period of these characteristics is the basis for the elaboration of maintenance strategies considering the three desiderata: the environmental issues, the vehicle’s reliability, and the operating costs of the vehicle.