Kinematic viscosity (40degreesC, 100degreesC), average M, coke value, and group composition were established in mineral oils (200SN, brightstock HBS), poly-alpha-olefins (PAO-6, PAO-40), and semisynthetic base oils made from PAO (SN/PAO-6 and HBS/PAO-40). Flory-Huggins interaction (x(12)(infinity) for binaries formed with 9 test compounds) and Hildebrand solubility (delta(2)) parameters, evaluated from IGC data, were found on oxidation: x(12)(infinity),; to rise most in long-chained hydrocarbon oils (HBS/PAO-40, PAO-40, HBS) and moderately or nil (200SN/PAO-6) in others, and delta(2) to fall most in HBS, PAO-40 and HBS/PAO-40 and slightly in others (generally, Deltadelta(2) less than or equal to 2 (MJ/m(3))(0.5)), thus indicating the composition and structure changes caused by oxidation to be large enough to prevent miscibility (e.g. of fresh with used oil).