It sort of looks like the bottle has been coated on the inside with oil.

Because of the chemical similarities between oil and plastics, oil really sticks to plastic, creating a hard to remove oily coating on plastic surfaces. You really notice this effect if you have the joy of having to hand wash a plastic mixing bowl that has had oil in it.

I wonder if something similar is going on here? If they are eating this stuff already is the magic ingredient simply an edible oil?

[edit] readability.