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.