Living in the midwest, we've had nothing but E-10 available to us for many years. I've done it both ways and from experience I can tell you it doesn't matter. Whether I've filled up or left it with around an 1/8 to 1/4 tank, the boat has always run fine the following spring. Just make sure to treat whatever gas you do leave in the tank with Sta-Bil. I'm to the point where I now leave about 3/4 tank of fuel in the boat when I put it away in the fall. This is so when we get the occasional 60 degree day in Nov. - Feb. I can break the boat out for a winter cruise and not have to worry about getting fuel.