I don't believe one can go past A.T.F. It is perfect for clutches, minimum drag, gets in where it's needed, plus it is an extreme pressure lubricant for gears, When I was working on Johnson outboards 135 hp. the instruction was if the OMC oil was not available for the lower leg gear set in your location use A.T.F.
Typical planetary gears in Car auto trans are small gears, small teeth and high tooth loading.
pancho.