I run 87 octane (E10) in my 1999 90HP Merc (& in my 2010 Tundra 4x4 Flex Fuel truck) ... and I buy Shell gasoline whenever I can. Reason being that I seem to get better milage/performance from Shell vs most of the other brands. BP gas is my second choice when Shell isn't available when needed.
I have run pure gas (E0) in my Merc a couple of times, but didn't see any major improvement. But, then again, I wasn't really monitoring it vs E10 performance at those times.
I started using Shell because I could get 10cents/gal off with my Kroger card. But recently the nearest Shell station to my home is now a Casey's and since I'm driving a different truck than the one I kept mi/gal check on and haven't driven it/filled it up enough (yet), I'm not sure if the Casey's gas will prove to be equal to my results with Shell (for the truck). I'll still stop at a Shell station when going to local lakes for boat fillups.