That would be "blow by". Some blow by is normal. It is the amount of combustion that bypasses the rings. If the amount is enough to actually feel you may have an internal engine problem.
breather tube from crankcase or valve cover brings blow-by to the air cleaner to reburn. excessive blow by will be heavy with oil in the air.
The engine is getting worn and blow by is getting into the crankcase and forcing oil from the valve cover into the air breather.
start with chnging your PVC valve it should go into your valve cover or into your air tube that's the cheap way
PVC valve
Valve Seals or Valve Train
Look on the valve cover. There is a small hose running from the air filter cover to it.
The front expansion valve is clogged.
It is what it is, one blows pressure out and the other simply releases it
Top of the left valve cover V8; about in the middle of the valve cover hidden underneath the air cleaner assembly. Back side of the left valve cover V6
Normal, you get a little blow back from the PCV valve system...
To release air that cause water hammer
On my 1997 Protege a hose runs from the valve cover to the air intake assembly. Where the hose connects to the valve cover is the PCV valve.