1982 Ford Mustang positive trac rear differential

It takes a 90 hypoid gear oil designed for Posi-trac differentials. You may have a hard time finding Posi-trac fluids these days and what we end up doing is adding the friction modifier additive to it. That can be purchased at most auto parts stores.

When you say positive track thats a GM play on words for Limited slip, which means you have both rear axles being sent power.

Which limitied slip do you have ? a Clutch disc style or a Helical style
The helical or gear driven style needs no special additives
the clutch style does need a additive

Where are you located ?

If you're in say AZ you may want to run a 90/110/140 weight oil to better deal with the heat.
If you're in MA, you may want a 70/80 to take better advantage of the cold.

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The manufacturer calls for a 90HP*(Hypoid Gear Oil for Traction-Lok Differential).

As I already stated, if you can't find any, use the friction modifier additive.

The title doesn't say Traction lok, so you're assuming nothing has been changed and is still a factory tract lok

Area and use can also dictate which weight could be used, as seen from the OE on tags on diffs when vehicles are used in different areas

If need be of a friction modifier order CM-19546-A1

If in fact it is using the TL and hasn't been changed