At the auto parts store, they sell that limited slip additive in a 5 oz. bottle (iirc) and I want to say it's called TransX. I always add that to my LSD and even did on my GTO, which called for a GM spec number; never had any issues. I know many of the oils claim to have LSD aditive built in, but in my experiences, it's never sufficient. The way you can tell is go on the freeway for awhile, get it warm, then go in the parking lot and do some turns...it'll feel like you're on square wheels
I'm not 100% on the scientific reasoning, but if I recall, the additive simply thickens it up to be able to cling and stay on the clutches and such in your unit. So, you'll be OK adding it, just don't go wild.