The change in the service properties of modern motor oils in contamination by fuel was examined. It was experimentally shown that mineral and synthetic oils can totally lose their working capacity when contaminated by fuel. The dependences of the properties of oils on the mechanical and thermal loads in the friction unit and the degree of contamination of the oil by the fuel were obtained. The limiting and damaging concentrations of fuel in oils of different quality were substantiated. Recommendations were made for determining the limiting concentrations.