With today's ethanol "enriched" gas, you definitely need to add a stabilizer such as Seafoam for extended storage to help minimize the gas separating and breaking down. Also, the fuller you keep the tank, the better off you will be. Ethanol will absorb moisture from the air and you will have water in your tank. With a full tank you have less air and therefore less moisture to be absorbed. I religiously use Seafoam in my bike because it sits a lot more than my Jeep. About twice a year I run some through the Jeep to keep things clean.