I don't know the answer to your question but I have changed from whatever was in the diff from the factory to synthetic in my 94 F250 4x4 and a 96 Chebby SS. Both vehicles have performed fine since the change and have at least 20,000 miles since the switch over.
Experience from the SS list has shown that if a synthetic is used, you should use more of the limited slip additive than you would for dino based.
On my SS, the rear differential chatters far less than it did with the factory oil.
This is my experience, but since failure can take maybe 100K miles, who would know if the change to syn caused failure. At least I can say in 20K + there is no sign of any problem and in one car performance seemed improved.
I think synthetic is recommended for heavy duty usage, such as towing. Synthetic is compatable with mineral, so I wouldn't worry about putting it in, even if it had mineral in it to start with. If it were mine, I would use a synthetic 75W-90.