Octane rating and compression ratio are usually tied together, but not always. There are many factors that influence the octane requirment, including the squish area, shape of the combustion chamber and valve timing. It is possible to build a high compression engine that uses regular gas, and some Toyota and Honda models do just that.
Pretty much every MB imported into North America over the last 20 years has required 91 or better octane unleaded gasoline.
jsap, with your car, just keep 'er filled with the highest octane you can get, and she'll be fine. If you have to put some 87 or 89 in, don't fret, just don't fill it, and top off with 91+ when you can. Your SL has a knock sensor and will retard the timing if it does detonate, but I personally don't like relying on them.
Some good info on octane in case anyone is curious...
(the higher the octane, the more resistant the gasoline is to "burning.")
http://auto.howstuffworks.com/question90.htm
And Bruce Hamiltons excellent FAQ on gasoline...
http://www.turborick.com/gsxr1127/gasoline.html