My previous car was a 2004 Corolla with 5MT. Over the 85K miles I drove in it I did 2 changes of transmission fluid, one at 40K and one at 80K. Per manufacturers specs, I used 75w90 gear oil, Mobil 1 synthetic. The car shifted and drove beautifully and at each change, the fluid came out dark in color but not burnt. Never had any issues with synchros, grinding, or missed shifts either before or after I changed fluid. I'd recommend the Mobil 1.