i get the highest octane from the local store, used to run 40:1 now run 32:1. if it gets 6 weeks old, it becomes weed killer, fire starter, parts cleaner, etc-anything buy fuel for my 2 strokes.
neighbor buys the 87 octane...and cusses and fussed about EEFANol gas all day long.
I suspect that the high-grade has less corn turd in it.
dino turd is preferred.
ALSO, i now always pour my fuel into a clear container to see what's at the bottom before fueling.
YES, fuel _can_ be hygroscopic.
http://www.fuel-testers.com/marine_boat_ethanol_problems.html
or let's say you do this: pour a teaspoon of water into "pure" gasoline, then pour a teaspoon of water into moonshine or whatever corn-likker you have handy...see the diff? one beaded and went to the bottom. the other mixed right in.
any vented system is exposed to atmosphere. atmosphere usually contains water.