All the previous replies are right on. I have used mid-grade in my 300E for quite some time. The key words here are "spark knock". If you hear spark knock, go back to the higher octane.

Good luck,

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Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles