'91 300-SEL owners manual specifies 91 octane gas. I live in an area where premium is rated at 93. I use it. Some here claim they have used the mid-grade successfully. I would not worry myself sick about the method used to drive the final number. MB said 91 and that's close enough for me.
If you want to asks questions about MBs, this is as good a site as I've ever found. Don't know about a website that describes vehicle features.