Gear oils are not all equal. If you have an older manual that uses a gear oil that requiers spec GL4 here is some info. GL5 is not better infact it may shorten trans life. The additives in GL5 are not compatibale with the bronze syncros. Info that I aquired states the use of GL5 in a GL4 recomended trans can shorten syncro life by 50%. It breaks them down. Not its hard to find GL4 but its out there. Good thing is most modern manual trans now use ATF or some other light weight oil.

Now onto ATF GM says if you trans is made to use DexIII the use of DexVI is not recommended. Again the new additives may not owrk well with the older trans. We are at a point in Hot Rodding that newer may not be better for older stuff. I would also try to stay away from "universal" products. If your adding power i would say not at all. If its a daily beater high miles I would go ahead.