The paper is devoted to the study of the regularities of changes in the quality of working fluids circulating through the systems of internal combustion piston engines. During the research, the parameters of fluids that characterize its operational properties, as well as external effects that can lead to changes in the properties of the fluid, were determined. It is shown that the deterioration of the quality of engine oil and coolant of piston engines during operation is caused by the destruction of additives that are part of the fluid and provide their required properties. A laboratory installation has been developed and created that allows simulating thermal and mechanical effects on working fluids similar to those acting during circulation through engine systems. The graphical dependencies obtained as a result of the experiment are presented. Mathematical processing of the experimental data allowed obtaining regression dependencies that correctly describe the processes of changes in time of the most important characteristics of working fluids under the action of cyclic thermal and mechanical effects. The described method can be used to study the influence of operational factors on the properties of various working fluids of internal combustion piston engines.