Gasoline that has set for that length of time will have begun to oxidize, and there’s nothing you can add to it to undo the process—the sludge will have already begun to form. Ideally, you would drain the gasoline, clean the tank out with B-12 Chemtool Fuel System Cleaner (part #0116), and use fresh gas. If that’s not an option, though, your next best bet is to use a combination strong solvent/high detergency product like B-12 Chemtool Total Fuel System Clean-Up (part #2616), both in stale tank and in the next fresh tank. if you have trouble starting the engine, try spraying B-12 Chemtool Carburetor, Choke & Throttle Body Cleaner (part #0117) into the throttle body while someone operates the ignition and gives the engine gas.