Whatever you want really. Rear end failures due to fluids are pretty rare. Any quality fluid will done fine assuming you don't have aftermarket parts with specific requirements.
I use conventional fluid...75w-90 iirc.....with the bottle of smell ford friction mod
Just overhauled my rearend back in july and thats what I used. No problems with it, used 2 bottles of rp with a shot friction modifier. Take note rear end fluid is some nasty stuff so try not to spill it or get in on yourself.
That's what I figured, I don't see how synthetic can really make much of a difference in the rear-end, I'm guessing (key word: guessing) it may even prevent the clutch packs from achieveing maximum traction.
Nothing special in back, just a diff from an 03' Cobra and a FRPP 3:55 gear set