My '91 300SEL owners manuals indicates an octane rating of 91 is required. More than one post @ this site indicates 89 octane is sufficient. Some believe it's even better than the premium fuel.
I suspect that local altitude comes into play as well. The higher the altitude, the lower the requirement. Some say if it's not pinging, use it. Mine purrs on 93 octane; wouldn't consider anything else.
1991 300-SEL - Model 126
M103 - SOHC
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