Just fill it up until the oil starts to flow out the filler hole then put the plug back in
and your good to go. Be sure to use the right oil as the Posi / limited slip is different and is usually has a tag identifying it on one of the bolts on the cover.
80W90 gear oil.
how much oil do i need and what kind
5 quarts
Depends on which differential you have but in most cases 4 to 5 pints should fill it.
SAE 75W90 synthetic gear oil
80w90 hypoid gear oil
Use an 80-90wt gear oil
The front differential uses 80W90 gear oil.
till it comes out of the fill hole about a quart
80-90 PL-5 gear oil
It is supplied with a convenient type of gear oil.
The only place you would use gear oil is in the rear differential and or front differential if it is a 4+4. The differentials have a fill plug.