I would like for someone who has an 84 or 85 500 SEL to look in there manuel and see what it says.
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I have an '86 560SEL and the manual calls for premium. I have used regular in the car and it runs fine with no knocking as the timing will adjust automatically to the octane level. But the car does run better and has more power with the "good stuff" in the tank and does get a bit better gas mileage with premium. I am sure your car was intended to use premium...best to stay with the factory specs.

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1986 560 SEL (159K miles)