I have a Kingfish presntly with a Mrecruiser 496. In Mass. we are all ethanol, during the season I don't use any additives but towards the middle of October when I figure I am getting close to my last tank I put in Stabil marine formula. When I winterize my mechanic gave me a recipe for a witches brew of Stabil, storage oil. Mystery OIl, etc. that I mix up in a gallon gas can and run the motor on. It is similar to the recipe in my Merc. manual but a little different. So far so good. Depending on your motor you may have various fuel filters that you need to attend to and I assume you have an inline fuel filter. I carry a couple spares for that but have never had an problem. The filters in my cool fuel module I change once a year. I'm sure you will get many different opinions. From what I read about outboards they appear to be a bit more finicky. If I ran an outboard I think I would use additives, with the I/O my program has worked fine so far.
