Gasoline is a very unstable fuel. Conditioners are used to keep the fuel from breaking down so quickly. I suppose to help keep the fuel from forming varnish and gum deposits. Also help the fuel burn better later. I've tried using gas both ways. In one generator, I've left fuel in for a year at a time and after around three years I noticed that the generator without the conditioner didn't run as well as the one I added conditioner to. Even though I changed the fuel and added cleaner, it doesn't clean out the varnish or gum buildup to help clean out the carb. I'd recommend adding the conditioner if you're going to store the fuel for any length of time. Doesn't hurt to add some RingFree also to keep carbon deposit form forming. Yamaha makes a excellent product. I've had great experience with all my outboards.