Gas without alcohol has a shelf life of a couple years. If it's a gas/alcohol mix such as we get out of the pumps it starts to degrade in a few months even in a sealed container. The alcohol has a great affinity for water. in short time under the right conditions the fuel will do a "phase-sepration". The alcohol/water will settle out to the bottom of the tank and the gas will float on top of it. Your pick-up tube will be drawing alcohol/water which may be what some are seeing as poor running engines, loss of power, internal damage caused.