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."
Your guy is out to lunch on both counts. 80W90 gear oil is a hypoid oil and contains sulphur, which is detrimental to brass synchros. That's the real reason not to use it. His second theory about synthetics being too slippery is equally bizarre. We have used it for over 20 years on everything from the current models going all the way to Porsche 356's circa late 50's without any problems.
