Date registered: Oct 2007
Vehicle: 2008 c300 luxury P2, Multimedia.
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well, if you read the manual, and your car happens to be one that can use either premium OR E85, than you should use one or the other...the manual specifically says that you should NOT mix these 2 types, and you should only "switch" between the 2 when your tank is less than 1/2 full..I plan on trying E85 when the tank gets down, but, it also says do NOT use E85 when the temperature is below 14 degrees F, which, here in chicago land will be pretty soon!
So, I would say whatever fuel you use, if you change grades, wait until your take is less than 1/2 full..