City transit buses are a severe environment for an automatic transmission fluid. The fluid must endure very high operating temperatures because of the use of brake retarders, frequent stop-and-go driving, and numerous shifts. There is an increasing trend toward the use of extended-drain, synthetic-based ATFs for such severe service applications. This paper documents a field trial with both synthetic and petroleum-based ATFs at a large municipal bus fleet in Southern California. Three different commercial ATFs, made with either API Group 2, 3, or 4 base oils, respectively, were compared after roughly 80,000 km. and one year of operation. Because of different additive packages in each fluid, not all of the results can be explained by base oil effects alone. However, the base oil is certainly a dominant contributor to the finished fluid performance. The following four variables were monitored by used oil analysis: iron wear, copper wear, viscosity change, and acid number change. The two synthetic-based ATFs showed directionally better performance in copper wear, viscosity change, and acid number increase compared to the Group 2-based ATF. There was no significant difference in ferrous wear protection between the three ATFs. The Group 2-based ATF is capable of providing at least double the OEM-recommended drain interval. The Group 3- and the Group 4-based ATFs can achieve 80,000 km drain intervals, which is four times the interval recommended by the OEM.