I was told a few years, I think at one of our MB clubs tech sessions, not to use additives such as Slick 50 in my 300SD. It was explained that this engine, possible other if not all MB engines inject oil into the bottom of the pistons when they are at the bottom of their stroke. This is done to provide cooling to the piston.
The Slick 50 won't cause any problems until after about 50K miles when it will begin to flake off the interior of the engine. These flakes will clog the ports which inject oil into the pistons.
Can any one confirm this and provide some more details.