You should use premium gas only because of the high compression of the motor. Using the wrong grade gas will cause preigntion and denotation of the air fuel mixture when its not wanted. You will be causing a lot of unnecessary heat in the piston area. Also when using the right grade gas you will greatly reduce the piston slap and prevent dieseling. Also you wont be buring all of your gas.

So bottomline you should use premium gas. Your mpg, power and operation of the motor will be better. The motor was desgined by engineers to use premium gas for a reason not because they felt like putting it there.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)