The effects of antiwear, EP, and friction modifying additives on the friction of steel-on-steel are greater when blended in more highly refined base oils. To explain this observation, non-hydrocarbons (sulfur, nitrogen, and oxygen compounds) of the types either (a) initially present in less well-refined oils or (b) formed by oil oxidation during use, were added to base blends consisting of a chlorinated wax, a sulfurized terpene oil, or didodecyl phosphite in a highly refined mineral oil. The resulting changes in friction and surface-coating formation were measured as a function of the nonhydrocarbon concentration. The additive effects on friction were canceled selectively by some of the 34 nonhydrocarbons studied at concentrations that could be encountered in less well-refined oils, whereas pure hydrocarbon additions had little effect. Mechanisms for the observed friction and surface coating changes have been proposed.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Chicago, Illinois, October 17–19, 1967.