I have been driving my '86 560 SEL since new, now with close to 150K on the clock, and use only regular unleaded fuel. As long as you are not doing a lot of high performance driving or experiencing any engine knocking you shouldn't have a problem. Others on this board say they get better mileage using premium but I don't think it is worth the extra 20 cents per gallon. My '86 averages 15/16 city and 18/20 mpg hiway at 70mph.

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1986 560 SEL (159K miles)