Were talking about read diff fluid and the only thing you would add is a friction modifier, nothing else. When he said additive I was thinking he meant some sort of additive like we'd use in oil (IE: lucas, slick 50 or some type of stabilizer). I guess your right, adding to something is an additive...How ever theres nothing you should be adding unless its a friction modifier to the rear end. Im not getting semantic, Im just being correct with tech support. In a non clutch locker you would not add friction modifier, nor would you add anyhting else (but correct oil)....If you dont like me giving correct support, start a forum and call it RenosPlace and Ill post incorrect data over there for ya.
To the OP, no FRICTION MODIFIER to your locking diff....or any oil additives.
EDIT: and BTW, some limited slip rear ends need FM, whether your friends tell you they like them or not is irrelevant.