The PCV = Positive Crankcase Ventilation and it needs to be kept cleaned or replaced but it is actually the less significant half of the ventilation system. A hose larger than the one on the PCV valve runs from the valve cover or a block vent over to the air filter housing. That vent system can get old and the hose collapse internally and the 90* elbows on the end connections can become totally plugged with carbon and sludge. When plugged the oil gets dirty quickly and buildup of sludge begins. That ventilation system is a significant reason that extended oil change intervals are possible.