Fuel storage

I know that over time gasoline changes and gets gummy (I've heard it called varnish, but not really sure what it is). I think it is due to oxidation. Can one of you more chemically savvy people explain what really happens to gas as it ages?

Also if it is due to contact with air, how long would gasoline last in a container that is vacuum sealed with nothing but gasoline inside (no excess air space)?