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
till it comes out of the fill hole about a quart
The front differential uses 80W90 gear oil.
Use an 80-90wt gear oil
A 1999 Chevy Z71 typically requires about 2.5 to 3 quarts of gear oil for the front differential and around 3 quarts for the rear differential, depending on the specific axle configuration. It's essential to check the owner's manual or consult with a mechanic for the exact specifications and recommendations. Always ensure you use the correct type of gear oil as specified by the manufacturer.
80-90 PL-5 gear oil
It would take four quarts of gear oil.