A water fuel separator would alleviate the ethanol fuel problem by about 99%. Racor makes them for a variety of motors. In my Yamaha F40 manual recommends min 86 Octane, no high test is required. I'd check with some of your local marinas to see which one still carries non ethanol regular. As for additives, I only use Sea Foam available at most car part places, CTC, etc. At the end of the year, I drain the tank, hose and get as much out of the motor as I can.