The steel-on-steel friction characteristics imparted by an oil-additive blend depend on the combination of friction-modifying additive, oxidation inhibitor, and base oil used in the blend. Increased refining of mineral oils generally enhanced the friction modifier effectiveness. The results with synthetic fluids generally fell within the mineral oil range. Free-radical inhibitor-type oxidation inhibitors enhanced or reduced the effect of friction-modifying additives whereas peroxide decomposer-type oxidation inhibitors did not. The magnitude and direction of the base oil-inhibitor interaction effect varied markedly depending on which friction-modifying additive was used. The results suggest that limited base oil oxidation may be essential for additive response.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Washington, D.C., October 13–16, 1964.