The effect of engine oil aging on the fuel economy of two matched 1998MY Buick Centuries equipped with 3.1L engines but operating on different GF-3 prototype engine oils (one SAE 5W-20 engine oil and a second SAE 5W-30 oil) has been determined in EPA FTP testing. Combined FTP Fuel Economy for these vehicles was reduced at a rate of 0.06-0.12% per 1,000 miles of accumulation. The data for the various parts of the FTP test indicated differences in the loss of FE with use for the two vehicles. The vehicle with the SAE 5W-20 oil containing a Mo-type FM additive showed a lower decrease in FE with use during the cold transient than the vehicle with the SAE 5W-30 oil. On the other hand, the vehicle with the SAE 5W-30 oil containing an organic type FM additive and a balanced detergent/dispersant package showed a lower rate of decrease of combined FE with use than the vehicle with the SAE 5W-20 oil. These differences may be indicative of the different additive chemistry in these oils. Used oil analysis indicated that after the accumulation of 8,500-8,900 miles, the oils had not oxidized to a significant degree and they had not reached the end of their useful lives. The lack of significant oxidation and thickening in the used oils is to a large degree responsible for the slow rate of FE deterioration. It is also indicative of the benefits of the advanced chemistry of the GF-3-type engine oils.