mid grade (89) should be fine, but only you can tell whether or not there is pinging as you drive. You didn't state where you are located - local climate can be a determining factor in the type of gas you use. In my 300E I would run 87 in the winter, 89 in the spring/fall, and 91 or 93 the few weeks of the year it was very hot.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280