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.
how much oil do i need and what kind
80W90 gear oil.
5 quarts
SAE 75W90 synthetic gear oil
Depends on which differential you have but in most cases 4 to 5 pints should fill it.
80w90 hypoid gear oil
The front differential uses 80W90 gear oil.
till it comes out of the fill hole about a quart
Use an 80-90wt gear oil
80-90 PL-5 gear oil
It would take four quarts of gear oil.
It is supplied with a convenient type of gear oil.