Either would work. GL 4 is for synchro transmissions. I don't believe the 2 speed axles have synchronizers. If they do, that would be the only reason to avoid GL 5.
So what is it about GL 5 and synchronizers that is not compatible?
I have been using and maintaining my 1958 and my 1963 Chevy C60 trucks for over 50 years. I'm sure I have added GL 5 to the transmissions not knowing better. I believe it was on this forum quite some time ago where it was stated not to use GL 5 in transmissions. That is what prompted me to ask this initial question about the differential lube.
So what is it about GL 5 and synchronizers that is not compatible?
