when changing oil in a pump that is not runing, I find that the oil level in the standing sight glass goes down when I put a plug or a vent on the fill hole. When I remove these from the fill hole, the oil rises in the sight glass. Sometimes, the oil rises and sometimes it lowers under the same circumstances. What causes this?

Just a guess. When you put in plug you trap and push down on headspace gases which are at room temperature and pressure, oil level drops slightly. If head space warms from oil or gases escape into headspace, headspace pressure increases and oil level is pushed down. If head space cools or gases dissolve into oil, headspace temp and pressure decrease and oil level goes up. When you remove the plug headspace goes back to room pressure and oil level adjusts accordingly. Or could be gremlins are messing with you.