Apples and oranges. Failure to use Dexcool will result in corrosion. Failure to use 91 octane when the engine is not pinging will do absolutely no harm. The manufacturer states 91 octane as an uneducated safeguard for the drivers who don't even know what pinging is.

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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