80W-90 GL-4 & 5 gear/hydraulic oil

[BarryinMN]

Does anyone know the difference between GL-4 & GL-5?

Everybody seems to stock GL-5.

 

[tracdoc]

The best place I've found for info such as this is the following website:
BOBISTHEOILGUY

I did a quick search of your query on GL4 vs GL5 and came up with a raft of threads. The one that seemed the best, albeit somewhat tech oriented is:
Gear Oil Specs

Hope this helps.