I've been using 87 regular in my '88 560 SEL. No problems. I've used 89 and 91/92 and no difference. I'm in NJ too, so our gas choices are alike. I know that the cluster states, Premium Unleaded Fuel Only. The comp. ratio 9.0:1 and initial timing, tell me otherwise.
I've considered removing the ignition timing resistor which will raise my initial timing and then I'll be using premium only.
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