I've been able to get away with running regular in my 300E for most of the year, up until a couple months ago when it started getting really hot around here (near 100). Then I started hearing pinging, as Duke said, at low RPM with heavy load - usually when cruise was on, on the highway. I switched to midgrade as the temps went up, then eventually to premium. In a couple months I'll be able to start working my way back down to regular again.

Regular is fine in my car when the temps are regularly about 80 and below.

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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