“Santner offers an additional storage tip: Keep that cap tightly closed. A loose cap–or even drum pump–can allow the vapors to escape, causing the fuel to go stale. As long as the container remains tightly sealed, he explains, any vapors will condense back into a liquid phase, maintaining the fuel’s freshness.”
The above is extremely important. The larger drums, 30 and 55 gallon drums may not always seal up vapor tight. That means as the temperature changes the drum “breathes” through the drum bung plug. If there is standing water on the top of the drum, it will get sucked into the drum when the weather cools off and your fuel is now contaminated. Make sure the bung is tight and the top of the drum is covered to prevent standing water accumulation. Personal experience.