Storing cooking oil

For my last camping trip to Cyprus Lake over a month ago, I thought it'd be smart to carry some cooking vegetable oil in an empty hot pepper glass bottle.  Today, when I took out the bottle to use the oil to coat the blades of my knifes, I saw some greenish brown spots at the bottom and on the side wall of the bottle.  What the heck?  How can mold grow in oil?  I ran some searches on Google and oddly enough, mold does grow in oil, or maybe it's because of contaminants from the air and what not that allowed for the mold to grow?  Well, I poured out the oil--it looks clean to the naked eyes--and tried to hot wash the bottle.  I poured really hot water into the bottle but nothing seemed to work to remove the mold.  It looks like the mold has taken root on the imperfect surface of the bottle.  As I cannot get a brush into the bottle, I had to throw away the bottle.  It is unfortunate because I like the bottle.  It is small and light for carrying cooking oil for camping.