In exchanges about changing/draining oil, when someone states that the pan is left to drain overnight or for a similar length of time under other circumstances, there is very often the rejoinder that this might be too long because the oil pump might lose its prime. What exactly happens in such a case? Why is overnight too long, but an hour or two is not? What's the worse thing that could happen? Aren't the pumps self-priming? Is the principal concern, then, the time lag for the oil pick up the prime while the engine is running at first startup? How long might that lag time be? Other considerations? Please help clear the air over this issue. Thanks all.