I turned my 91 S-Class over to a detail/trim shop for this, and they actually used a heat gun to warm the adhesive (they have heat guns that get hot enough to do this, but not hot enough to scorch or blister the paint), and the emblem came off like a dream. It did take a little polishing over the next few weeks to get rid of the "ghost" of the emblem (you could see "300SEL" in the paint in certain light), but now the paint is glossy with no trace of the designation. I used Meguiars exclusively, FYI -- their Deep Crystal 3-step method.

By the way, the shop charged only $5.00 to take the emblem off. I'm assuming you mean the model number only, as my S-Class 3-pointed star is mounted to the trunk via three holes. That one stays!