The Chilton's repair manual states, if you have a limited slip differential there is a warning plate attached to the differential and that only special gear oil should be used.

The warning is in German, I don't have the book with me now, but if anyone is interested I could look it up at home and put on here later. Seems that my 1984 300D can use synthethic gear oil. Now I have the hard time of making a choice!