Engines always have PCV and gasoline contamination to deal with... Differentials might move air in and out of them in the grand scheme of a cup a day. Automatic transmissions are a closed loop and virtually never see any outside contaminants... manuals don't move much air in and out of them either. Due to shearing and heat they usually recommend the fluids other than the motor oil be changed at 20,000 miles.
I'm 99% sure the reason why you change engine oil is due to contaminants... I know I never get 3,000 miles out of my oil before it's contaminated.
But anyway, I am planning on running 80w90 in the 465. The slingers and stuff seem to be set up for gear oil. I know the guts on the various T5s (I've got a lot of experience on them) changed with changing requirements of fluid. I also know that some transmissions (like Chrysler's AX5 and AX15) like a specific kind of fluid for different years or else they shift poorly and wear fast (usually require GL3).