It really depends on where and how the barrel is stored. If is kept somewhere cool and is 100% sealed, it should last quite a long time. It’s when it’s kept unsealed and the temperature varies widely that it goes bad, mostly from the constant expansion and contraction and the associated moisture that is pulled in because of this. In theory, if it can’t expand and and contract due to temperature changes, it shouldn’t absorb moisture or evaporate the light vapors that keep it fresh.
“when it’s kept unsealed and the temperature varies widely that it goes bad, mostly from the constant expansion and contraction and the associated moisture that is pulled in because of this”
Also, gasoline is a mixture of several hydrocarbons. The more volatile will evaporate more quickly, leaving the less volatile ones behind. Those less volatile ones don’t combust as readily as the lighter ones.