Pulling vacaum through oil filler tube.

[PKRWUD]

There is a filter underneath the PCV valve in the intake, and if it is not replaced every so often, it actually encourages oil to "hang out" in it, which makes it easy for the oil to get sucked up into the PCV valve, and into the intake. In a similar light, aftermarket intakes often require a baffle be installed there, but it rarely is, which also encourages oil to be sucked into the PCV.

Take care,
~Chris

P.S. While designing your PCV system, remember that unless the air that enters the crankcase first goes through the MAF, it will be unmetered air, which is the same thing as a vacuum leak. Just incase this last part isn't fully understood, allow me to elaborate. Air enters the crankcase through the oil fill tube. It circulates through the crankcase, picking up air contaminated with blow-by gasses, and exits the crankcase at the PCV valve. From there, it goes to a vacuum port on the intake, and ultimately into the combustion chambers. Since it is air that ends up in the combustion chambers, it needs to be accounted for, thus, metered, by the MAF. That is why the factory source for this air is at the TB. That way, the air has already been filtered and metered before it enters the crankcase, and ultimately, the combustion chambers.