I remember something about GL 5 should not be used in an application that has brass or copper fittings or parts because the higher level of EP additives in GL5 (vs GL4) can cause corrosion to the brass and/or copper parts in the gear box. The brass is most likely to be used for the synchro mesh components? i.e. GL4 is most commonly used in manual transmissions and GL5 in bevel boxes and differentials, especially hypoid gears? I can't remember.