A series of engine oils belonging to various viscosity classes 10W/40, 15W/40 and 20W/50 (SAE) and quality classes (API) were examined. The oils of an identical viscosity class (produced with the aid of various base oils) were characterized by similar values of basic physicochemical parameters (viscosity index, density, refractive index). These parameters are significantly different from those found for the 5W/40 oils. The SIMDIS HT 750 analyzer (ASTM D 6352) enabled the type of the basis oil used for the preparation of the respective engine oil to be determined. The Flory-Huggins interaction parameter and the Hildebrand solubility parameter were used to estimate the mutual miscibility and solubility of the engine oils examined.