I find "gasoline" stations near lakes where marine engines are used. I find that the reason is that gasoline survives storage better than ethanol.....otherwise why the stations located where they are?
On 87 or 91, my Merc. OEM manuals state minimum octane is 87. My engines run on 91 including my small engines at home. No doubt, call it old wives tale or hocus-pocus, or superstition, they start better, idle better, and just run better. 91 runs around $275 which is cheaper than ethanol premium which likes the $3 number most places selling gas, even in this fuel glut where 87, 10% or less ethanol is running around a buck 40.
If you want to complete the job, get some NGK iridium plugs (cross referenced to what you are running in conventional plugs).... NGK catalog available online, and really notice some improvements....ebay sells them.