Original PVC systems had a solid cap and drew clean air out of the air cleaner with a tube to the valve cover, through the engine to the PVC valve and then to the intake manifold, a closed system as Tim mentioned.
Most conversion PVC systems draw fresh air in a vented oil filler cap(not air cleaner). If that is how yours is set up I would say your PVC valve is not working or is in backwards, blowing oil out the cap. The filler cap should stay clean for many miles unless you are in dusty conditions.
