This article discusses the effect of changes in the viscosity properties of motor oils on the operation of engine parts. The quality of engine oils, especially viscosity indicators significantly affect the reliability of the engine, fuel consumption and other parameters. Therefore, high-quality selection and use of motor oils is very important. Operational factors, such as the speed of starting the engine, pumping oil through the lubrication system, cooling of rubbing parts and cleaning them from contamination, are most dependent on the viscosity of the oil. The coefficient of friction depends on the viscosity, and consequently, the reliability and efficiency of the machine, aggregates and friction units.