All jokes aside, this got me curious to look for the definition of "oil", a word used to describe stuff extracted from plants, rocks and, apparently, animals too.
It turns out that for a liquid to be classified as oil, according to Merriam Webster, it needs to be "flammable", "not dissolve in water" and "greasy".
What's greasy, you ask? Well, it's smeared in... "oily matter".