Why does cooking oil "dry out"?

I don’t know if I’m phrasing this correctly. Sometimes a trickle of oil remains on the outside of my oil tins, and over time, it becomes gummy and kind of “lacquerizes”. At this point, nothing on God’s green earth will clean it off.

What’s happening to the oil? There’s no water in it, I assume, so it’s not evaporating, is it?

Oil *does *oxidize.
I once used three-year-old crisco :eek:
Had to throw out a batch of fried chicken! :mad:

Because it’s a Drying Oil.
Whatever oil you’re cooking with is unsaturated enough (contains enough double bonds) that air oxidation leads to the formation of a hard polymer.
Your use of the word ‘lacquerizes’ is apt because many drying oils are used to finish wood; Flax (linseed), Tung, Walnut, Safflower. Corn and Sesame oils are semi-drying, i.e. they tend to form goo, while Olive, Peanut, Grapseed and Palm, are non-drying. A nice table of the drying properties of various oils can be found here.