GL-5 is for hypoid gears, so it doesn't need to also be specified.
The nasty smell is sulfur and phosphorus compounds used as anti wear additives. This is what reacts with yellow metals like synchros, so there is much less in GL-4s. Hypoid gears require much more anti wear properties, so GL-5 has more sulfur and phosphorus.
The Amsoil should be fine in most differentials. If I had it, I'd use it. I don't think I'd shop for it due to the cost. Maybe you'd need more LSD additive.
There was a building at the Pennzoil plant in Oil City called the Old Barrel House. It was a brick building where kegs and pails had been filled with gear lube for almost a century. I'll never forget the stench of that place.