Do you really need fuel additive?
I have lot of seasonal equipment and used 7 yo diesel just fine.
Gasoline is more tricky, but I had boat with 3/4" open vent and even after 2 years the gasoline would start the engine, although with excessive smoke.
My Honda lawnmower runs on gas from previous season just fine, but I always shut off the fuel and run carburetor dry at the end of season.
In some climates water condensation is the biggest issue, but I have never seen it in CA.
Seafoam is mostly kerosene.