Worm oil is DINP (diisononyl phthalate) or DOP (dioctyl pthalate), both of which are a pthalate used as a plasticizr. it's not a liquid oil in the traditional sense but a chemical compound that is mixed with the plastic resin during manufacturing.
From what I understand it is basically the compound that is used to make the plastisol finished product softer ( more oil )or harder ( less oil ) Much as hardener is what we see as the hard pack in the bottom of the jugs our plastisol comes in. Is the white part a PVC compound?
Maybe this is right, maybe I oversimplified it?