In a few cases the differential gears are made so light (cheap) and carry so much load for their size that the highest quality lube is the only one that gives them a chance to survive. In cases like this the auto maker will precisely specify which gear oils will work. If not exactly spelled out in the owner's manual, yes, any good name brand gear oil of the correct viscosity and other specs will likely do a fine job.
Riz, get the gear oil specs from your owner's manual or any good auto parts store can look up the specs in their books. Any name brand should do fine. If indeed the spec is for 75W-140, then it has to be synthetic to get that wide viscosity spread. Any name brand is fine. The other spec will be GL-5, which simply means that it is suitable for hypoid gears which just about every differential has, and just about every auto differential gear oil meets the GL-5 spec. (Hypoid gears have one shaft in a different plane than the other shafts...drive shaft below the plane of the axles in cars...with the result of much sliding of the gear teeth, and the oil must contain additives that permit this without wear.)
Im no expert, but every 30,000 my diffs get drained and refilled with whatever is on sale... ive owned many autos and never had a issue with this approach.
I am gonna second that, Ive tried three different brands, all synthetic, all 75w90 and I could tell no difference in noise, gas milelage or anything. Whatever I get a deal on.
yep, go into the zone, whatever 75w90's (or correct spec for my application) is on sale is what goes in it.. and frankly with this approach ive never noticed any rearend noice, ect,ect.. and to be real honest most of what comes out usually looks almost as good as the new stuff going in. Ive never been able to tell (seat of the pants) any difference after i change gear oil..