GL-5 to rear diff and IRD

Hi all,

Im planing to use Castrol Syntrax Longlife 75W-90 GL-5 on IRD and Rear Diff. However Ive been reading that GL-5 have addictives that are more "aggressive" to yellow metals as copper than GL-4. But I think for IRD and Rear diff should be a problem correct? However for Gearboxs should be GL-4...

This make sense?

cheers
LT

Use what the book says GL 5 iirc