Typically you'll want to keep a full tank of gas. The idea being that it leaves very little "air" in the tank to allow condensation to form (and get water in the gas).
It should be noted, however, that in cold climates (like NY, or MI) the winter air is usually pretty dry.
I usually fill it up before putting it away for the winter. This allows for the above reasoning, but also gives me a full tank of gas for when I occasionally run the engines.
Fuel stabilizer and a full tank of gas. Make sure you run the generator for about 20 minutes after adding the fuel stabilizer. This allows the fuel stabilizer to circulate through the generator's fuel system as well.
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Dave
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054