It should be noted that 'diff' oils are rated differently to engine oils and at 100C 80w90 is something like 5w30 engine oil in viscosity (This is an approximation on my part, not an absolute)

The GL5 oils are meant for hypoid gears - i.e. most diffs. The 'difference' is that with the pinion centrline offset from the crownwheel centreline (like inside a diff assembly) the gears don't just engage against one another like in a gearbox, but they actually 'slide' across the face of each other. This requires something particularly special in terms of chemistry to help keep the oil inbetween the crownwheel and pinion teeth. And believe it or not, that oil can actually be corrosive to other metals. I've heard it described as it 'etches' the surface of the gears so that the oil has something to cling to, but I don't know if that is accurate.

What I can attest to is that diff oil is (to varying degrees) potentially corrosive.

Since you don't have hypoid gears in a gearbox (well some transaxles might) you don't need the gl5 spec, and it can actually be less than ideal.

Plenty of cars from the past specced diff oil for their gearboxes. Yet others specced the same oil for the engine and gearbox.

My hunch is that they both 'work' but were a bit of a concession to practicality. If you can run the same oil in the diff and gearbox (or alternately the engine and gearbox) then it's one less 'specialty' oil that you have to have in stock either at home or at a professional workshop.

Generally no harm will come following factory fill advice. This was not the case in certain local fords many years ago, as they specced a multigrade oil for engine and box. Problem with that is that the vi improvers and other additives in multigrade oils were particularly vulnerable to shear, which will happen aplenty in a gearbox, and basically the quality of the oil in a gearbox filled iwth multigrade oil will deteriorate rapidly (especially if highly stressed, like in a modified car or any sort of hard usage) so it will be 'well' below the intended spec.

Some (if not all) modern synthetic oils are stable across a range of temperatures, and don't have a (vulnerable) additive package that goes off and stops working (generally the vi improvers of old 'uncoiled' and kept the oil from thinning out too much at higher temps, whilst still being thin enough to pump/flow during cold starts in winter climates)

SO WHY THE HELL DID I BOTHER WITH ALL THIS?

Well if you want to pursue other oils, you can look at some monograde engine oils (which are still readily available) or some other oils that are essentially a sort of engine oil (or a variation on it), but not a gl5 spec diff oil. TO this end, I can only speak very highly of shell donax TD - which was available in both 5w30 and 10w30. Of which the 5w30 had a slightly better additive package. It's a specialty oil that was run in a bunch of drivetrains for heavy agricultural gear (the kind of equipment that costs more than most of our datsuns combined!). I know of someone (it's how I found out about it) who has in his lifetime worked at high end racing teams, and run/maintained various fleets, from transport through to farmyard gear, who swears by it, and uses it in every applicable situation (from the t5 in a commodore, to various tractors, or dozers, I forget which, and even the ride on mowers!)

Now the only 'hitch' here is that approximately 6-12 months ago, shell started to rename/alter their product lineup. So I don't know for sure what it is called at the moment! But I'm pretty sure if you call their tech line, they would tell you quick smart.

It used to be available in (from memory) 20 and 200l drums & I think 5 or 10l.

I don't have any left, I went halves with someone one a bigger container. So if you don't have any luck in finding the new name, I'll probably find it out in the next few weeks anyway!

It's pretty good stuff. Works well in the toyota superchargers if anyone cares.