Jim, the original fluid would be 80W90 gear oil. The change interval from Ford would probably be 100 -150,000 km.
Your car does NOT have a plate-type traction-lock diff, the 94 - 95 auto. trans SC cars were equipped with an open differential, having the electronic traction control tied in with the ABS brakes instead.
All of the 89 - 95 manual trans. SCs, and 89 - 93 auto. trans cars, had the traction-lock differential that uses the friction modifier with the gear oil to keep the clutch packs from chattering.
I've used Ford 75W140 synthetic gear lube in mine since just after I got the car at 55,000 km, I have almost 180,000 on it now. Ford had a service bulletin for many of the cars in the mid-90s with 8.8 locking diffs, to cure a chattering problem. Mine would groan and chatter when hot while making tight turns, in a parking lot for example, after driving it for 30 - 40 minutes. The bulletin advised the use of updated clutch packs for the diff., the friction modifier, and the 75W140 synthetic fluid.
cheers
Ed N.