A lot depends on the climate you live in, but that being said, no two ways about the fact that it has an affinity for water, and that makes it corrosive - period. I would not leave it in my $700 carb for a month ( much less over the winter )without running it. Gas stations - ALL E10 and E85 tanks will have some amount of water in them. If the tanks are not sticked for water and the water sucked off regularly, the water level gets to the bottom of the tank pickup, and guess what you get. Newer tanks ( even E10 ) have electronic monitoring systems just for that purpose. It will reek havoc on fuel injectors also - don't ask me how I know!!!