But its not a closed loop. An auto engine operates at temperatures up
to 235 degF because of the pressurized cooling system. Water in the oil
boils and steam escapes from the oil. The PCV system effectively sucks
the moisture laden air, steam, and oil droplet mist in the crankcase
into the intake manifold. It gets mixed with other air that came
through the air cleaner and fuel, gets compressed in the cylinders,
goes bang, and 95% of it goes out the exhaust pipe. Only a tiny
fraction leaks back into the crankcase as a result of blowby...

The process reaches an equilibrium with very little residual water in
the oil. When the engine is stopped and cools, that is when the water
content of the oil is the lowest.