Polyunsaturated oils combine spontaneously with the oxygen to deteriorate into a hard film.

Vegetable oil bottles tend to get sticky on the outside. Used vegetable oil is great for wood preservation. Many stains have heavy metals in them.

https://en.wikipedia.org/wiki/Drying_oil https://en.wikipedia.org/wiki/Lipid_peroxidation https://en.wikipedia.org/wiki/Lipid_peroxidation#Hazards