Consider that no seal is perfect. Valves, valve seals, and piston rings all leak at some level. Combustion by products leak past the rings and fill the crank case. If not vented, this would pressurize the crank case blowing valve covers and robbing power. Other very bad things will happen if not vented (water condensation in oil for starters).
These vapors are usually vented through the valve covers. On the Mustang, fresh air goes in the left side, combustion by products are drawn out the right side via the PCV valve.
The crank case vapors are a mixture of oil, unburnt gas, and combustion by products. Normally these are vented via the PCV value and drawn into the intake for burning. The PCV valve ensures that only a vacuum is applied to the crank case. It also prevents any flame (back fire) from passing through it.
Sources of excess oil include missing PVC baffles, worn seals, worn rings, worn/leaky valves. The oil ends up in the intake because as it cools, the oil can condense turning from a vapor back into a liquid. Since the oil is heavy and does not easily evaporate, there it sets in the manifold.
Over filling can cause this. Removing the baffles behind the PCV can also cause excessive oil to be drawn in. A sticky PCV valve is more likely to casue venting problems not excessive oil.