This video shows an unbonded tube. The 'oil' is exuded from the rubber edge gasket. While the rubber is still flexible on an old tube, the surface is not clean, but covered in this sticky substance, and may carry some black material from the rubber. I'm guessing that they were made with an excess of "plasticizer." In the case of my tube, there was some residue on the inside of the cover glass that never came completely off. So, there must be some volatile substances given off also. Not visible under normal operation, but still visible when the cover was off and reflecting a ceiling light. The gasket at first was covered in black goo, which I suppose was a combination of deteriorating "rubber" and plasticizer. It came off on my hands when I picked up the lens. After the first cleaning, I made the mistake of setting the lens gasket side down on the cabinet, where it immediately left a dark circle in the wood finish.