In this paper, studies of the thermal-oxidative stability of mineral and partially synthetic aviation oils for aircraft with piston engines are carried out. Method of research of lubricating oils for thermal-oxidative stability in the temperature range from 160 to 180 ℃ is described and includes the application of testing devices and control, such as a device for thermostating of the oils, photometer for direct electrophoretic oxidized oils and electronic scales. The parameters of the optical density and evaporation of oxidized oils at temperatures of 160, 170 and 180 ℃ were determined. On the basis of the obtained results, the dependences of the optical density and volatility on the test time and temperature and the dependence of the decimal logarithm of the time when the optical density reaches the value D = 0.8 on the test temperature are constructed and regression equations are obtained. According to the obtained regression equations, the critical temperature of the oil and the potential resource, the temperature of the beginning of the oxidation processes and the evaporation rate are determined, characterizing the thermal and oxidative stability, which expand the information about the temperature range of the operability of the studied oils.