Originally Posted by MTI

Pressure under the valve cover is vented via tubing to the air cleaner housing. There are "baffles" in the valve cover that are supposed to separate most of the oil particles from the vented gasses.

As the engine ages, valve seals and guides can deteriorate and allow the engine's compression to pressurize the area under the valve covers, increasing the amount of oil that will get past the baffle and into the air filter housing.