Kohler, Briggs & Stratton and Westinghouse write their manuals independently and disagree about plenty. They do not disagree about how long gasoline lasts. Briggs states gas begins to degrade in as little as 30 days, which can lead to the fuel system clogging and the equipment not starting the following season. Westinghouse states gasoline left as little as 30 days can form gum, varnish and corrosive deposits that block fuel passages. Kohler simply instructs owners not to use gasoline older than 30 days. Nothing wears out over a winter - the fuel changes chemistry while it sits.
Under 1 month: Westinghouse states plainly that no service is required. 1 to 2 months: add stabilizer and run the engine 2-3 minutes to circulate it through the carburetor. 2 to 6 months: Kohler treats 2 months as the threshold for the full storage procedure; Westinghouse says fill with fresh gasoline, add stabilizer and drain the carburetor float bowl. 6 months or longer: Westinghouse drains both the tank and the float bowl; Kohler and Briggs still do not call for draining provided the fuel was treated.
This is the one point the manufacturers do not agree on. Briggs & Stratton stores the mower with a full tank of treated fuel and is explicit that using stabilizer eliminates the need to drain. Kohler adds PRO Series fuel treatment and runs the engine 2-3 minutes, and also does not call for draining. Westinghouse drains progressively - the float bowl at 2 to 6 months, the tank as well past 6 months. The distinction worth taking away is that the tank and the carburetor float bowl are two different decisions: the bowl is a small reservoir where evaporating fuel concentrates into varnish that blocks the main jet, and it is where the spring no-start is manufactured.
Both Kohler and Briggs put the oil change in the storage procedure rather than the spring startup, and Kohler specifies doing it while the engine is still warm - used oil holds combustion acids and moisture that otherwise sit against bearing surfaces all layup. Kohler exempts engines filled with KOHLER PRO 10W-50 full synthetic. For the battery, Kohler says disconnect the negative cable; Briggs says negative first then positive, stored cool and dry away from heat sources. Kohler also specifies fogging the cylinder: remove the spark plug, pour about 1 oz of engine oil in, reinstall and crank slowly to coat the cylinder wall against rust.
Westinghouse adds the step that is a safety instruction rather than a longevity one: let the generator cool at least 30 minutes before storing, and store it in a well-ventilated dry place away from any pilot light, furnace or water heater. A gas water heater in the same garage as a fuelled generator is a pilot flame a few feet from gasoline vapour. Kohler also states that E15, E20 and E85 are not approved and should not be used, and that the effects of old, stale or contaminated fuel are not warrantable; up to 10 percent ethanol is acceptable.