Don’t kid yourself about mothballs. I got wiped out by a lady in a model-T using premium gas. What I didn’t know was that she was running a funny car with a 456 hemi. However, those were the days of lead/unleaded gas and before ethanol entered the scene. Ethanol can be used to raise and meet octane requirements. Premium gas may or may not contain ethanol depending on the requirements of the state that you’re in. Bottom line: I use what the manufacturer recommends to keep the warranty, and clean out my carburetor at least once a year.