I've been doing some research. It varies by state, but Michigan allows 10% ethanol in gasoline without any notice at the pump. Ethanol attracts water. It also "phase separates" from gas when sitting unagitated. The ethanol/water mix sits on the bottom of the tank. When we warm up our engines we are burning off this non-gas mix. That is as bad, or worse, than the effects of ethanol on soft parts such as fuel lines, gaskets, etc. Additives do nothing to nullify the alcohol/water damage.
This site: http://www.pure-gas.org/ lists service stations, airports, and marinas, in the US and Canada, which sell ethanol free gas. I found 4 stations in my area that sell 91 octane ethanol free gasoline. Current pump price is $3.39, which is cheaper than the $3.99 at the marina or $5.05 at the airport.
I haven't been able to find any reason not to run 91 octane in my old tractors, so I will be using it from here on out.