The paper describes the results of research on changes in the performance of fresh and used semi- synthetic motor oil ELF Evolution 700 STI for gasoline engines of cars. The change of physicochemical properties of fresh and used semi-synthetic motor oil before and after its use in a gasoline engine has been studied. The results of derivatographic and IR spectroscopic studies of these oils are presented. The composition of the inorganic part of the studied semi-synthetic oils was determined by the method of X-ray fluorescence research.