Oil is the medium of time manipulation magic. Created through ancient sacrificial rituals, it is is a substance that can be used to create aging/rot-retarding barriers, or refined into derivatives that increase the rate of plant growth and mechanical work. To be handled with care, as extended contact can lead to corruption of the body, as well as increased susceptibility to fire elemental spells.

  Simple rituals can render an inferior product from most living things; the time-manipulation abilities of such substances will be weaker, but the substance will be safer to handle, and can even be imbibed (this is a double-aged sword, reducing one's vital life force while increasing one's bodily proportions to that of a toddler).

You either see it or you don't.

There's actually a useful and quite generic metaphor to be excavated here. I would just tell you what it is, but I think you'll more enjoy finding it for yourself.