bad rear main seal??
Never, ever under any circumstances put any additives in your oil. None of them work as advertised, and may actually cause damage.
+1 on checking the plug. It's easy while you're in there. Actually, I'm pretty sure that GM sells the entire rear main cover with the new seal already pressed in and installed as well as the gasket for the rear cover. Or if you want to you can just pick up the RMS at an auto store and do it yourself. Lol, I'm lazy so I just bought the rear cover already set up. While you're swapping it out just check the plug. Pull it out and check the O-ring on it too for wear or damage. It sits behind the rear main cover on the drivers side.



