As others have said, the car will run on regular or mid, and not be harmed but fuel mileage could / would drop off.

My 97 SL320 with a 3.2 L inline 6 has less power on mid grade so premium is required. ( heavy car )

My 97 C280 with the 2.8 L inline 6 ( same basic engine as the 320 ) runs just fine on mid grade however, I've never checked MPG. I keep this car for Mom and Dad so it sees sub 50 MPH round town driving, if it was being run on interstates I'd run on premium as engine load would be much higher.