Originally Posted by POS VETT

If it was strictly and merely GL5 in transmission, it would be about the synchro gears being unable to synchronized the gears quickly (I.e. shift a lot slower and with revv matching to avoid grinding), but the sulfur-based additive in gear oil might do some damage chemically on the synchro gears that are made of softer metals.

GL4 in the diff is likely okay; it's less slippery than GL5, but it shouldn't hurt anything.