It depends on what “Extreme Pressure” additives the manufacturer put in the oil to get the GL5 rating. The lower cost alternative is sulfur compounds. The nonsulphur compounds are more expensive.
As I said above, the primary application of “yellow metal”, aka, copper alloys in drive train components, is as synchronizer rings in transmissions.
I guess if a manufacturer was trying to cut costs in the front axle they could put in bronze bushings as opposed to bearings, and possibly make a crush sleeve to located the pinyon gear out of bronze. But, both would have very short service lives so, I would not expect them to be used.
You can hunt around for GL4 lubricant if you want to. I researched the topic, pretty extensively a few years back, when I first bought my tiny truck, and it specified GL4 for the transmission, transfer case, and axles.
As a result I have a pretty good understanding of what things are, and how they work together, and I have no hesitancy in using “yellow metal safe” GL5 of the appropriate viscosity, in anything specifying GL4.
The difference mechanically between the GL3, GL4, and GL5, is how well they function under high pressure shear as the gear teeth move across each other under “extreme pressure”. GL3 has less EP capabilities than the GL4, and the GL4 less than the GL5.
