Untreated gasoline that's allowed to stand for over a month may form a varnish on the inside of the fuel tank, carburetor, and other fuel system components. Draining the gasoline reduces varnish problems.
Please be aware that even when the gasoline is drained or run "dry", small amounts of fuel will remain in the fuel system. If the fuel is untreated, this small amount of fuel can still lead to problems. Whether the fuel is drained or run dry, it is recommended that stabilized fuel should be run in the engine until it is warm to ensure that treated fuel is circulated throughout the entire fuel system. Please note:
The Environmental Protection Agency recommends adding the drained gasoline to your car's gas tank, provided your car tank is fairly full. Once diluted, old gasoline will not harm your car engine.
Another way to protect against the varnishing effects of stale fuel is to add a gasoline stabilizer such as Briggs & Stratton Advanced Formula Fuel Treatment & Stabilizer to your fuel before storage. This allows fuel to safely remain in the fuel system and helps prevent gaskets and seals from drying out.