Your car has an M103 motor - 6 cyl. There have been posts here and on another MB lists that indicate mid-grade gas(89 octane) being adequate for these motors. I too have an M103 motor and have always used 93 octane premium. Personally, I wouldn't use 87 octane in one of these motors. My 2 cents.

Mike Murrell
'91 300-SEL