The results of the comparative analysis of the physico-chemical properties of the fresh and used motor oil are offered. It was found that the level of pollution, waste engine oil mechanical impurities exceeds the allowable, and aggregate stability of the colloidal system is connected with a high content of additives that preserve the dispersant. The study of the dielectric characteristics of the used motor oil revealed that the main products of the "aging" are soot particles. Using the data obtained is recommended in the selection of conditions and compositions for the purification of waste motor oil.