I'm sure I've read that gauge blocks left wrung together become very hard to separate, and it's taught that it's bad to try to pull the blocks apart – they should always be slid apart. In both cases I believe that the surface can be microscopically damaged. I suspect that the microscopically thin oil film, which would be very difficult to remove, at a molecular level, and any trapped gas molecules act to make separation easier, by preventing cold welding-like effects. Surely inter-atomic or inter-molecular forces are responsible, when such welding-like behaviour occurs.