Topical aspects of assessing the effect of crankcase gases of an internal combustion engine on the operational properties of engine oil taking into account its service life are considered. For a mathematical description of the change in the condition of engine oil, a complex criterion for assessing oil operational properties and a criterion for assessing the saturation with crankcase gases are proposed. At the same time, it has been found that the change in each of the criteria depending on the service life and engine rotational speed can be approximated by an exponent at a confidence level amounting to 0.95. The parameters of the proposed criteria have been assessed. It is shown that these criteria can be used jointly for diagnosing the condition of engine oil. It has been established preliminarily that the detailed service life of the engine affected by the properties of engine oil can be determined by the simulated time of interaction between the oil and the crankcase gases.