Remember that if you burn a pound of avgas you get half a pound of water. We have all seen water dripping out of a car exhaust on a cold morning. Combustion products blowing by the rings enter the crankcase where they come into contact with relatively cool surfaces and condense. Early Porsche motors were famous for making "Motor Mayonnaise", an oil and water emulsion that would collect in the stem of the oil tank.

Letting the crankcase vent after a flight is a good idea. It is a better idea after a short flight where the oil has not been hot for long, after low altitude flights where the vapor pressure is low relative to the crankcase pressure or in humid climates.

Opening the oil cap will cause air to flow in the crankcase vent and out the filler due to convection. Most of the crankcase vapors will likely be displaced in the time it takes to secure the airplane. Secure the cap and close the door when ready to leave. If you have to walk away with the cap off, flag the oil door with a "Remove Before Flight" banner.

Brent Regan