[hatzie]

I've heard just below the fill hole but I just fill it til it runs out. There's a special Kent-Moore SPX J-tool dipstick for the later model T800 & later light truck axles because some genius at GM decided the bottom of the fill hole needed to be above the actual oil level instead of the way it was for 60 or so years before.

80W90 GL-5 dinosaur juice in the front and 75W90 GL-5 dinosaur juice in the rear. The owners manual or service manual will have the dry fill amounts. It's somewhere around 2 quarts or a bit more.
No limited slip additive needed and absolutely don't use it if your RPO tag lists G80... unless you know there's an aftermarket limited slip diff installed in there.
Don't bother with synthetic oils in these axles.

Avoid gear oil that says Energy Conserving on the API seal if you have the optional GM Eaton G80 locker.