If someone cut a retired road draft tube canister open they would find a series of baffles meant to separate the splashed oil from the crankcase from the blow-by gases and condensation the PCV is intended to eliminate. As an engine wears and more combustion gases find their way past the piston rings, the PCV can begin to ingest oil and other contaminants it's not supposed to handle and become restricted, or clogged. At that point, the only exit vapors will have is through the fresh air intake pipe, resulting in an oily residue collecting in the air cleaner housing. On a badly worn engine from the mid-1970's with that faux filter in the air cleaner housing, I've found half an inch or so of oil accumulated in the bottom of the housing and actually being sucked through the paper filter element. Those engines could put a mosquito fogger to shame with the blue smoke from the tailpipe!
Jerry
