Base fluid-metal interactions of a super-refined paraffinic mineral oil were studied. Wear debris from a Shell Four-ball wear tester was analyzed. Based on the amount of iron distributed among used oil, organic solid, iron particles, and iron oxides, one is able to interpret the wear phenomenon in terms of mechanical and chemical processes. Oxygen concentration at the wear junction controls the type of the chemical interactions between the metal and the lubricant. These chemical reactions appear to be controlled by the thermal gradient at the junction; catalytic effects by the metal are insignificant.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Kansas City, Missouri, October 3–5, 1977