[JimD]

five_timer,

When the PCV valve opens because the pressure exceeds it's setting, it is letting gas out. That gas obviously had to get in. It is probably gas getting past the rings. That sort of ventilation is a lot worse for the motor than ambient air that may come in while the oil is changed. The point is simply that car engines are not completely sealed systems. Ambient air is one of the least damaging things that get in.

If there is something else I said that you do not understand, please let me know.

Jim