In certain types of city bus service some automatic transmission fluids can fail in less than 10,000 miles. In order to provide satisfactory transmission performance for longer mileage, improved fluids are required.An investigation was undertaken to obtain improved fluids. Fifteen different fluid formulations were evaluated in 30 city buses operated in normal service for more than 2,000,000 miles. It was determined that fluids fail because of frictional deterioration and oxidation. Based on these evaluations, only two fluids were found to be satisfactory for more than 40,000 miles; one additional fluid was satisfactory for more than 30,000 miles. The remaining 12 fluids failed in less than 20,000 miles.From this fluid evaluation program, it is concluded that with properly formulated fluids, drain intervals in city bus automatic transmissions can be as great as 40,000 miles; however, drain intervals should be established by evaluating fluids in the bus service for which they are intended. Analysis of fluid samples taken during a test can aid in establishing the optimum drain interval.