I just bought a rebuilt G52 5 speed manual trans for my 1985 Toyota 4X4 pickup and the shop guy said to use 10W-40 engine oil in it, NOT 80/90 gear oil. He said when it's cold outside and the gear oil is cold, if you push the shifter hard you can actually break the aluminum shift fork from the resistance of the thick oil.
He also said NOT to run synthetic because it's actually too slippery for the synchros to work right.
My Chilton manual recommends GL-4 or GL-5 75/90 or 80/90 gear oil.
I'm going to use the 10W-40 because the shop will warranty the trans with that oil. They wrote on the side of the trans case by the check bolt in sharpie "10W-40 motor oil."
The syncromesh and redline type fluids are designed specifically for manual transmissions even if they are synthetic, that mechanic is old school and has no knowledge of new lubricants.
