If the fluid you drained out is clean, just dump it back in. Otherwise, any kind of trans fluid is okay. Rossi used to sell 10wt motor oil as racing fluid. I always change the filter. Never run the motor to warm up the trans with fluid level below the full mark. Folks are scared to death to overfill their trans even a quarter inch above the full mark but you have to be several inches up to damage the unit. Running the motor with the fluid low enough for the filter to suck air will destroy the trans. Two quarts low will do that easily. If you need to drain any out, just use the neutral switch.
