Try Marvel Mystery Oil, 1 oz. to 2 gallons of the crummy petrol they sell us now, and the fuel will never sour even sitting for 6 months over my winter here. To be totally safe I use 1 oz. of MMO, and one ounce of Stabil winter gas treatment, and fill my 2 gallon tank. It will run fine after 6 months of just sitting, with the petrol in the tank, but you can drain that petrol out, and do all fresh stuff. Use the old gas in the lawn mower. I use 1 oz. MMO in every 2 gallon fill up, even all summer long. I have never had the fuel turn to sludgy brown varnish on me, I have seen a friends sportster gas turn to brown sludgy like mush in 6 months in his bike. What a mess, carb had to be torn down, and totally cleaned, the tank had to be cleaned with solvents and B B's, or old studs from studded tires. Save your studs when they wear out, pull them out of your tires, and use them for cleaning things that you can not get your hands inside of.