If nothing else, it makes no sense to me to use an oil that is optimized for friction (ie synchro rings); in an application where friction is the enemy (like a hypoid differential).
FWIW, I've been using Valvoline Synpower (GL5) in all my differentials for many years (over 20 years in my previous TR3A, current TR3 hasn't been on the road that long since I got it). The 3A diff was already well worn when it came to me (with over 150,000 miles on it), and I credit the Synpower for making it last while I put another roughly 200,000 miles on it. Was still working fine (in spite of all the backlash) when the car was wrecked.
In spite of the GL5 rating, the Synpower has none of the additive that attacks "yellow metal". It doesn't even have that distinctive "old gear oil" smell when I change it (the smell being some sort of hydrogen sulfide compound related to the sulfuric acid that attacks yellow metal).