GM's "Limited Slip Additive" is a "friction modifier". It reduces squeal and/or chatter by degrading the friction that provides the limited slip function. Consequently, you want to use only enough additive to eliminate the noise or chatter and no more.
GM recommends to simply pour in a bottle. While that makes sense in the service trade environment, it doesn't make much sense for DIYs interested in best limited slip performance and "best limited slip performance" may come with just a slight amount of noise...not chatter, mind you...but a little bit of squealing.
There are two issues one needs to address before using the additive:
1). The diff needs to be in good condition. If there is significant chatter when the car is driven at slow speed in tight circles, it is likely that enough additive to reduce or eliminate that problem is going to degrade limited slip action so much that the diff is not very effective in limiting slip. Significant chatter likely means you need to overhaul the rear axle, not add another bottle of limited slip additive.
2) You must use a lubricant formulated for use in clutch-type limited slips. There are several good choices out there, but if you don't have the correct lubricant to start with, you may defeat the limited slip action you need.
My personal choice is Red Line Heavy Shockproof Gear Lubricant along with 1 to 1.5 oz of Red Line Friction Modifier. I use this in all my rear axles, regardless of vehicle type or duty cycle and I've been using it since the early 1990s. Heavy Shockproof is notable for it's reddish-pink color and its unique, calcium-based EP additive package. HSP was originally formulated for off-road truck racing, a duty-cycle known for very high driveline shock-loads. It's also become quite popular in nitro-class drag racing, again because of the performance of the calcium-based EP additive package in duty-cycles which subject ring-and-pinion sets to extreme shock loads. HSP works well in clutch-type limited slips but, typically, a small amount of RL Friction Modifier is necessary. Note that Red Line's "Friction Modifier' is not the same as GM's "Limited Slip Additive." Both are friction modifiers but their formulations are vastly different.
The way I end up with just enough Friction Modifier is to fill the axle with RL HSP to just below the bottom of the fill plug. Then I road test the car beginning with 5 miles at highway speeds to warm the axle lube.
Once the lube is hot, I find a parking lot and drive in tight circles at slow speed, clockwise and counterclockwise. If the axle squeals or chatters, I add an ounce of Friction Modifier then I road test again. If the axle still squeals loudly, I add another 1/2 oz and road test. Typically 1.5 ozs does the trick and I am satisfied if the axle squeals modestly in tight circles. I trade a little noise for better limited slip action.
If the squeal is still too loud I'll go another half to a full ounce but much more than that indicates there is likely a problem with the limited slip that an additive is not going to solve.
