Do not allow the Avgas to be run in a non aviation engine. Good chance that vehicle's systems would be damaged
I would happily run many year old Avgas in an aircraft, as long as it was free of water, and properly filtered. Avgas does not "go bad" or change characteristics the way Mogas does.
Simply, Mogas is a mixture of many different liquids, each with their own properties necessary for porper operation of the engine. When the "light ends" evaporate away, the Mogas is changed, and generally the engine won't start well.
Avgas, on the other hand is only one liquid, so as it evaporates, what is left does not changes characteristics. You know how Avgas will evaporate from your hand, leaving only a dry white residue. Mogas when it evaporates, leaves a greasy film, which is the liquid in the Mogas which does not evaporate, and that's what gums things up.
So, consider checking the condition of the Avgas, and running it in your plane, I would....