[Gold Country Russ]

IF the PCV orifice has not been modified the only remaining route to have excessive vacuum inside the valve cover or pan is thru worn out intake valve guides. Whether the intake valve is open or closed there will always be less pressure (vacuum) inside the intake port and manifold than inside the valve cover so if the valve guides and oil seals are worn out oil and air will be pulled into the intake port. Generally there other symptoms when the guides are worn which can include poor MPG, surge or misfiring at low RPM, rough idle, poor power, and finally P0300 to P0304 codes.
Because bad valve sealing is a result of bad valve guides you need to do a compression test to see if you need new valve guides and a valve job. See my currently active post "Lets talk about leaking valves" to learn more about these engines. It is my opinion that all '04 & '05 and most '07 & '08 engines will need the heads repaired long before the engine actually wears out.