Does anyone know which type of gear oil I should use for refilling my rear differential after I drain it?

1990 RS, TBI, 305 engine.

My car is not posi q.q

 

Honestly, it doesn't really matter if it's an open diff. Factory is probably 80W-90, but I'd do 75W-90 synthetic

 

From the numbers on the axle housing, it says my camaro is a Standard differential and not posi/ limited slip.

But I want to make sure som1 didn't change the inside gearing to posi,

What is a good test to do to find out if it is STD or Posi?

 

with the rear wheels off the ground and trans in neutral turn one wheel. if the other wheel turns the same direction its posi, if it turns the oposite direction or not at all its an open rearend. if you just jack up one rear tire and the trans in neutral wheel should not turn at all. open rear will turn the drive shaft with the wheel.

 

Thanks!!