This is what we use at the volunteer fire stations.
https://www.trufuel50.com/4-cycle-mix/

Much of the smaller equipment gets pulled out and tested every 6 months, but unless its a busy fire season, very little regular use. We also keep cans on the trucks of both the regular and premix, then you have no evap loss. Once a fire season starts up, we will switch to pump fuel and mix as necessary, but at the end of a season we tend to drain out and put this stuff in till the next year.

Its pricey, (about 20 bucks off the shelf) but if you find a hardware store that carries it, you can negotiate to buy in bulk. They have gallon cans of the regular stuff. I think our cost comes out to be about 9.50 to 10 a gallon. We may be getting a fire discount as well.