I use Penrite oils in my box and Diff,
the Redline Stuff is great too, but pricey, the shockproof stuff gives a bit of life to 300zx box's..

I use the redline lightweight shock proof in the box, made a good diffence.

Shell donax TD 5w-30 for gearboxes. It's good enough for some really heavy duty agricultural equipment/drivelines. Should do the trick. Might be hard to get in less than 20L drums though. there is also a donax td 10w-30 - but I've been advised the 5w30 is the go - not so much a weight/viscosity issue, but the 'hidden' difference is the 5w30 variant has a different additive package and should be better.

Diff oil has to be different due to hypoid gears being in use, so it's not as good to use it in gearboxes (it's also slightly corrosive, and a post on another list I'm on indicates this is on purpose as it does some sort of surface etching/application that helps it to adhere under higher pressures, but it isn't as good for parts not designed for it.

Anyhoo - on the diffs - for a friction type lsd - mobil SHC ID (maybe listed as SHC 80W-140 ID) - their 'heaviest' spec synth for diffs.

For open or torsen style diffs - mobil SHC 75W-90 or SHC 80W-140 ID - either would be ok, and both are syntetic.

you'll be in for a bit of a wound to the hip pocket with these, but considering you'd safely get 40,000km per oil change on the diff oil at least.....