To the best of my knowledge, MBZ limited slip axles such as the one on the 16-valve use a conventional clutch type limited slip differential, and the friction modifier additive is required if you use a conventional mineral oil based gear oil. The only gear oils that I know of that don't require the additive (because it is already added) are some synthetic blends from Redline.
Go to a GM dealer and order 1052358. This is a four-ounce bottle of the friction modifier for clutch or cone type limited slip units. If your diff capacity is about two quarts or less I would suggest starting out with half of it, but don't quite fill the diff in case you have to add more. If your diff capacity of 3-4 pints, use the entire 4 0z.
After you complete the job, drive the car around for a week and see if you detect any chatter. If you hear/feel any chatter, you'll need more additive. As a final check for chatter, find a parking lot where you can drive the car slowly in a tight circle. If you can't detect any chatter, you're good to go. If there is some chatter, you'll need to add a little more additive.