custom90 wrote:

Always GL4 in gearbox, GL4/5 transfer box, GL5 diffs.

Thats what I go by anyway, mineral EP90 is thick compared to fully synth.
Uses more fuel, not so sure mineral is so good with seas either such as the pinion oil seal.

Personally I would not be overly worried, just so long as GL4 is used in gearbox of which that’s a different topic anyway.

There is other factors such as anti foaming agents and things like that.

I used to use mineral but now use fully synthetic, better properties in the cold as well.

Mineral being thicker perhaps has a little more protection, but when you will be paying for that in fuel all the time by that loss of MPG you’ve probably bought a coupe of diffs anyway.

Perhaps worth checking the materials the lockers are made with hence the bronze component issues, I don’t think standard diffs have any bronze components but are hypoid gears etc.
I’m not an expert though, don’t profess to be but I wouldn’t go back to mineral personally but that’s just me.
If the component manufacturer explicitly state a specified grade though I’d stick to that, it’s probably due to the lockers components if that is the case.