Performance-based test techniques were developed to determine high- and low-temperature automatic transmission fluid viscosity requirements. High-temperature fluid viscosity requirements were determined using engine-transmission-dynamometer tests. Low-temperature fluid viscosity requirements were established with a motor-driven 3-speed transmission apparatus. It is concluded that the current used-fluid Dexron viscosity limit of 5.5 cs minimum at 210 F provides a good safety factor for preventing excessive internal leakage at high temperatures. Since some Dexron fluids have -40 F viscosities approaching the 55,000 cp specification limit, lowering it will be considered in future specifications.