The rates of the antioxidant consumption in a synthetic oil used in various engines were found to be normalized when plotted vs. the product of engine firing events and the surface area of the oil film per cylinder. This suggests that the antioxidant consumption occurs primarily in the piston-cylinder area of an engine. The pulse flame generator was used to determine the specific effects of combustion products on the antioxidant decay. The rate of antioxidant consumption in a synthetic engine oil has been followed as a function of flame-oil distance and the concentrations of nitric oxide, carbon monoxide, and unburned hydrocarbons in the combustion products of isooctane and air. The results suggest that the diffusion of free radicals formed in the fuel combustion event into the oil film is an important process in the antioxidant decay in engines.