I'd much rather use a GL5 in a T/case but some (usually cheaper) GL5's can cause copper/brass/bronze discolouration due to the EP sulphur additive not being buffered properly.
To be sure, check the oil's copper discolouration rating on a PDS, but the more expensive GL5's should be fine in a t/case, and the good synthetic and semi-synthetic brews can often be dual rated if blended for manual gearbox use.

