I am about to re-install the engine and transmission in my 91 RS after some work. When I pulled the T5 out, it was filled with gear lube. The case clearly says in big *** letters fill with Dextron II ATF only. The trans worked fine when it was pulled, should I drain it and refill it with ATF or top it off with gear lube?

 

I have run both in my T5. It had gear oil in it when I bought the car, and had replaced it once with the same. The last time I pulled the engine, also changed the gear oil out in the tranny for some ATF. The T5 is considerably more quiet with the ATF (not to mention easier to shift when cold), which it was designed to use.

 

There may be a reason the trans had gear oil in it. How long have you had the car? The gear oil might be quieting the trans due to looser gear tolerances.

 

Its true that the heavier gear oil might be hiding noise and comforting worn synchros, but bearings and gears will starve to death with this oil in the case. It will not shift easily when cool. It requires more torque to turn, which equals more wear on the engine and the trans, and poor fuel milage. The funneling system is not adequate enough to throw heavy oil where it needs to go. It will not flow to the 5th gear set in the rear of the trans. Use GMs Synchromesh oil or a mixture of ATF and motor oil of light viscosity. If the trans makes ugly noises...fix it.