Rear axle seal leak

Look at the RPO list in the glove box, If you see G80 on the list you have the GM limited slip axle.

You want API 80W-90 around 2.5 quarts. You fill it till it to 1/4" below the fill hole.

Now as to the additive, GM is on both sides of the fence with the G80-

Eaton who (make the G80) strongly recommends utilizing an API-GL5 approved Mineral-Based 80W-90 Axle Lubricant and one 4-oz bottle of Limited Slip Additive

BUT GM says in TSB 76-02-02A
All other limited-slip or locking differentials including C/K (NON GMT 800), S/T, M/L,
G trucks
Use only GM Part No. 1052271. Do not add limited slip additive!

I generally defer to the folks who make the parts and run the stuff they say to run, I have seen and heard units that ran the synthetic from GM chattering and slipping.

I will also say that the G80 is called the Grenade 80 for a good reason. If you do not drive with a heavy foot and sort of baby it, it works fine. But if you get stuck or decide you want to really get on the gas, and light up the tires, it will not last long.

OK no G80 means you can use any conventional or synthetic oil API 80W-90 rated. Or synthetic 75W-90 Both will work just fine. I only run synthetics because it gets cold here and the synthetics seem to flow better in the cold.

As to brands, I use two most of the time Mobil and Amsoil but the Valvoline should be fine. The others are probably just as good, as they could all be the same manufacturer.