If you are running non ethanol fuel, add a good quality marine stabiliser. The fuel does lose some of its octane rating, but with the preservative, it will last over 5-6 months. Most marine fuel (marked ) is 91 octane rating while most outboards run fine on 87 octane fuel (check your manual). In the past I mix 50/50 ne4w mixed fuel with the old fuel and have encountered no problems. Use a good fuel/ water separating filter and drain off the water after each trip, until you find no more water or have gone through a tank or so. With ethanol fuel, you have a very good chance of having water accumulate, so you may have to drain the filters more often. I usually leave my portable tanks about 1/2 full over the winter, before topping up before the first trip of the year. I once left a tank of fuel for two years and all the fuel had evaporated, leaving a very slimy mess on the bottom, of 2 stroke oil and stabiliser.