Molecules can permeate plastics. The smaller the molecule, the more likely the permeation. Sometimes they add a barrier layer to packaging to prevent permeation. For example, potato chip bags often have a shiny, metallic appearance inside because they use metalized plastic (look up aluminum deposition, if you're curious) to prevent oxygen from permeating through the bag and making the potato chips rancid. I seem to remember reading about the newer plastic gas tanks on cars using multilayer plastic, where a barrier layer prevents (slows) VOCs from diffusing out, to meet emission standards.
That said, myself, I wouldn't worry about motor oil.