It's my understanding that these items were actually functional, rather than purely commemorative/advertising. They were placed under the lid of the sealed drum of oil, in such a way that they would come loose when the seal was broken and the cap was removed, and attempting to replace the cap with the medal still in place would cause the medal to fall out of the cap and drop to the bottom of the oil can. That way, when the purchaser opened their freshly boght drum of oil, they could be reassured that their oil drum had not been tampered with and that they would in fact receive the full volume of oil they had paid for; no-one in the supply chain could siphon off some of the oil from out of the drum prior to their receiving it, without disrupting the seal and removing the medal.
Being exposed to motor oil, either through direct contact with the oil or by the vapours, sometimes for prolonged periods, is the reason why these medals often take on peculiar colours and toning patterns.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis