Just to add to the equation, the 84 and 85 117 engines (maybe others) call for 87 oct. I used premium fuel for years and the car ran good. When gas went over $4 one time, I switch to reg. Guess what, I could feel that the car actually ran better. Fuel mileage was up a little also. Now to go deeper, the engine came out of a car made for the Mexican market (long story). The CR is 8.0 to 1 as far as I can find out, while the US version is 9.0 to 1. So what I have concluded, the German car makers know what they are doing. If they say Premium, use it. If they say Reg, use it.

Paul

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84 500 SEL (307,xxx miles)