Fuel absorbs water from the air, and the methanol tends to evaporate faster, so you may not see a large weight change initially.

I've had guys show up with fuel jugs with about 1/2 gallon of fuel, which had gone a week uncapped in a cool location, that could not start their engines with that fuel.

Once I flushed out their tanks and engines with fuel from a newly opened jug, no problems what-so-ever.


On the other hand I've seen people panic when they see a bit of condensation on the inside of a sealed jug I have...


Took the fuel, shook it, popped the seal, filled the plane, flew all day.