Tomguy,
If you have a U.S. version 4.5, it was designed to run on regular. If you are having a problem, you need to take a look at your advance mechanism or the computer or something. I was new car prepping these cars at the dealership at the time they were introduced. I even run a 560SEL and a 90 300SEL on mid grade without problems. I also run a 90 300E on mid grade. Admittedly if I really wanted to work these suckers, I'd fuel them with Chevron premium. The 4.5 is a low compression engine and should run fine on regular.

Peter

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