the big problem of GL4 and GL5 is the addatives.
alot of the GL 5 oils have phosphorus anti whear properties.. this is BAD news for soft metals.
I use GL 6 in my tranny. and YES its OK.. it does not have the adatives that make things go all bad on ya.
(it works out to be SAE 150 )
I also use 80/90 in my T- case.. NO worries there. Don't put ATF in those things, they only did this on some of them. there is no reason to. If you stay with Dino oil, use pensoil Syncromesh. and put the same thing in your t-case.
and unless your me, use 80/90in your diffs.
I use 85/140 in my diffs, but thats a whole nother can of worms. (aftermarket diffs)