I would try and keep the tank full if the tractor has to set for any period of time. Check with your local parts store, they may have some additive you can add to the tank to help with the moisture issue. Also I would buy the highest grade of gas you can buy.
My experience with this fuel, it can ruin old engines, many will just not run on it.
Others say different, but take your time, do not put it in all of your small engines at once.
Get on line any goggle non- ethanol gas, there may be a station that still sells non ethanol gas.
Good luck with those old motors.