My experience with limited slip additives has been that they are all pretty good. The GM, MOPAR, and other, branded additives are all excellent; you will pay a premium for the name. I use the additive that meets the required specification for the diff I am playing with. If it is cheap, that much the better. Gear oil is a matter of personal preference, but you will get better quality with a name-brand oil, in most cases. Sometimes, the "store brand" is a re-labeled premium brand, so look carefully at labeling. For instance, NAPA's store brand oils are made by Valvoline's producer, Ashland Oil. I use an 85W140 weight oil in my differential, and would recommend a thick lubricant for any abusive environment, like racing, or off-road play. It does sound as if you probably have a sick unit on your hands, and may be in need of new clutches, or a new carrier. Look for a lot of metal in the removed fluid- that would be a bad sign. If the issue is an incorrect fluid, or the lack of the proper additive, then there is usually a "ratcheting" sound associated with the lack of limited-slip operation. If it is quiet, and doesn't function normally, the chances are that it is time to tear it down.
