Dynamic friction and static immersion tests in which oil temperature was varied and dynamic friction tests in which load was varied have been run on steel balls with oil blends containing seven commercial additives. Analyses of the ball surfaces by X-ray fluorescence have shown that significant surface coating formation is initiated at calculated surface temperatures of from 250 to 350 F for most of the additives studied. Although the surface coating “thickness” generally increased with temperature, the rate of increase was generally different for the static and dynamic tests. Both surface temperature and mechanical shearing appear to influence the reactions responsible for the surface coatings. Used oil analyses showed that base oil oxidation, additive decomposition, and, in some cases, sludge formation became detectible at temperatures corresponding to the start of significant surface coating formation. The results suggest that in situ polymerization may be an important phenomenon occurring on rubbing surfaces.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in San Francisco, Calif., October 18–20, 1965.