I used to keep oils around forever, and somebody once said something about oils going bad over time. I hadn't really noticed anything, but being a foodie and a chemist, I got curious.

Now I wish I had never learned that oil can go rancid. You cannot imagine what it's like to live in a world where oil is rancid but most people can't tell. I don't think there are any true negative health effects, but now that I can detect (smell and taste) rancid oil I see it everywhere. My parents and parents in law both have 5+ year old oil containers that they use maybe once a year.