Oil would blow by the rings, stem seals, rotating seals and eventually, gaskets if there's too much pressure. The oil cap seals the system. That's why you feel something coming out because the natural movement of pistons pressurizes the crank case (which is why you have a PCV) That's how most cars work anyway. Some nissans you have to pull the dipstick out while filling them to vent the crank case since filling closes off the oil cap filler. Likewise those cars you better not pull the stick out while running or boom lol