The oil is not under pressure. The pressure under the valve cover is the same as in the crankcase, which you stated was above atmospheric. The pressure in intake port is below atmospheric. So if you have 3 psi gauge pressure above the intake valve seals and negative 15 psi gauge pressure below in the intake port the 18 psi pressure difference tries to equalize past the valve seals and takes some of the oil vapor under the valve cover with it.