One problematic additive is ethanol, often found as 10% of gas pump fuel volume. Ethanol absorbs water vapor from the air, then settles to the bottom of the gas tank or carb bowl, and corrosion or deposits may ensue. Drain tank & run engine until carb is dry, or use canned fuel, or add stabilizer like SeaFoam and fill tank to the very top. Ethanol-free fuel is available, but may be hard to find, see: https://www.pure-gas.org/