MB limited slip diffs from the eighties are clutch type - just like GM's classic positration, which was first offered in 1957. The clutches have preload, but with use they wear, which reduces the preload and it eventually behaves just like an open differential. Most manufacturers publish a minimum preload torque, which can be used to determine the health of the unit. A subset of the clutch type is the "cone type" used for light duty applications. Instead of a clutch pack, the differential side gears are tapered and bear against a similar taper inside the differential case.

Clutch and cone type limited slips need a "friction modifier" to prevent "clutch or cone chatter", which is a form of stick-slip friction that can make the diff noisy or jerky on tight turns. Most of the OEs sell a limited slip additive (basically all the same), and it should be added to straight 80W-90 GL5 gear oil in the proper concentration.

Replacing the limited slip clutch pack is probably a pretty pricey operation and may not be worth it on an older car. If the clutch pack is worn the differential will continue to function as a normal open diff with no ill affect other than loss of the limited slip function.

Since clutch wear particles get into the oil I recommend diff lube changes every 30-60K miles for clutch and cone type limited slip diffs, however, on a open diff, after Mercedes' recommended break-in change (not sure if they recommend this now, but they did in the eighties), it's not necessary to change the oil on an open diff.

Normally, with about equal traction on a slippery surface, both wheels will turn at equal speed, however if one wheel has traction and one doesn't sufficient input torque can overcome the preload and allow one wheel spin by overcoming the clutch clamping force, and this can also quickly burn up the clutches. Clutch type limited slips are "torque sensative" in that they respond to axle differential torque. They are usually defined by "percent limited slip", and a "30 percent limited slip" would "lock up" at a differential torque of 30 percent. Differential torque creates an end thrust on the differential side gears, which adds to the preload to create more clamping force to "lock-up" the diff. Street cars are usually set up with about 30 percent limited slip. Higher values can make them cranky when going around corners - chatter and jerking.

Torsen limited slip diffs use gears and are based on the principle that a worm can drive a worm gear, but a worm gear cant drive a worm, and they respond to differential torque. One advantage they have is no internal wear items and no need for a lubricant additive.

Another type of limited slip is the "viscous type". This type has a silicone fluid sealed up in the differential case whose viscosity varies with the rate that it is sheared. This type is sensative to differential axle speed, not torque, and, like the Torsen, there are no special maintenance requirements.

BTW, I have never understood why the German OEMs charge so much for limited slips diffs. On both my '63 Corvette (clutch type) and '76 Cosworth Vega (cone type) the option cost was between 40 and 45 dollars. I don't recall Mercedes even offering the option on US models from the eighties (my 190E 2.6 could sure use one), but it was standard on the 560 and 16V. BMW offered optional limited slips on some models in that era, and the cost was about 500 bucks!

Duke