I also have a 200 gallon motor oil tank in my shop that has been vented to the air for 25 years that gets filled about every 12 months. So a little humidity does not hurt the oil. Water and oil do not mix.
In 25 years we have never had any moisture issues in our bulk motor oil tank and we pressure wash it clean on the outside some times along with the whole shop.

So what would happen if the plastic container lets in small amounts of air and moisture?