My owner's manual (2000 320ML-215hp) states 91 as the min. Of course many gas stations in the Eastern US sell 87,89,and 93 as their octane choices. So, you may want to cut back 1-2 beers a week at your local bar and buy the 93 octane.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback