I use Seafoam, 2 oz per gallon for 2 cycle engines, 1 oz for 4 cycles. I don't drain out gas for the winter. I have never had any problems, but has is never around for more than 8 months. It's cheaper than buying non ethanol fuel.
Key point. If rad inlet hose is hot, check the air temp coming out of the fan (watch fingers). If there is a "goodly" amount of heat coming out, the coolant is flowing properly. If no heat out of back of fan with rad inlet hose hot, then there is low or no flow.