on 99 and up models unless you have an SS or firehawk (and possibley the WS-6, im not sure) with the HD auburn option, you dont have a limited slip. they come with torsens now, they are not limited slip, they use gears to performe the same basic function, but they are not by definition limited slip. additive isnt necessary, but reccomended to keep noise down

 

that is a good point although zexel and gm term it as a limted slip, it works more as a cheap locker and not that good a one at that.