rear diff. oil

I have a 2000 Z71 (with a limited slip diff.) that has a very slight leak from the rear diff. cover. I want to replace the gasket and fill with new oil or something better than what came in it. Today the dealer priced the gasket at $7 and the oil at $28 per bottle and said it would take 3 bottles to do the job. I've read some posts, and hear a lot about additives and using other brands of oil.

If I use another brand of oil do I need the additive? If so where can I get the additive, and about how much does it cost? If I can use another brand of oil and don't need an additive about how much does a quart of the other brands of oil cost? Has anyone just used another brand without additive, and how did it work?

Any advice is greatly appreciated. Sorry for the long post and multiple questions, but I want to be sure, and would rather not have this become a $100+ fluid change if I can avoid it.

Limited slip diffs do require a certain oil to work correctly . Those must be small bottles . Most of the bottles in the parts stores are 28 -32 ounces . I changed the fluid on 77 chevy diff and it only took about 3/4 bottle . I don't recall whether my bottle list limited slip compatible . Rob

1999 Silverado limited slip diff - just had it serviced by a local mechanic - he put the limited slip additive, it cost me a rear in. Starting with the Silverado's you are not supposed to put he additive, the rear end explodes. It is in the service manuals. It takes almose 3 quarts of 10W -90 synthetic.