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In the latest passenger car motor oil specifications the Sequence IIIG engine test is used to determine the ability of lubricants to control piston deposits. We have analyzed the chemical composition of Sequence IIIG deposits in order to determine the source of the piston deposits and determine if the mechanism for deposit formation in the Sequence IIIG engine test is similar to previously published mechanisms for formation of high temperature engine deposits. These previous mechanisms show that combustion by-products react with lubricant in the piston ring zone. The mixture of combustion by-products and lubricant are oxidized to form deposit precursors which are further oxidized to form deposits. Since the Sequence IIIG engine test uses lead-free fuel it is important to reexamine the nature of piston deposits formed in gasoline engines and in particular in the Sequence IIIG engine test.