There was a good thread on this two months ago. I'm not able to provide the link, but if you search a posting by 95E320cab on 9/5/02, there's a link to a good car & driver article which addresses the issue of burning regular in a premium engine. In a nutshell, you lose in gas mileage what you save at the pump for premium burners that have knock sensors on the engine.
If you're used to changing your oil every 3000 miles, continue doing it for your MB. The logic for doing this on other engines also applies to MB engines.
95 E320 Cabriolet, 131K