As said above it's normal on a worn engine. On a good engine there should be a slight vacuum.
It tends to be blowby from the piston rings pressurising the crank case.
Compression/leak test your engine. You could run like it for years if its only worn rings, but fix it if you can afford to.
When it gets really bad it can start blowing oil through the breather system into the intake and give clouds of smoke out the exhaust. This will most likely happen after you top the oil level up or slightly overfill it.