Do you live in a area that has cold winters? Do you have injectors?
A Cold winter starts are the likely culprit. Engines require a significantly richer fuel/air mixture to start in cold weather. A high percentage of the fuel that's delivered from the injectors and vaporized into the air drawn into the engine condenses back into liquid on the cold internal engine components, including the valves, pistons, combustion chambers and cylinder walls.
The extra fuel supplied for startup ensures that enough fuel remains vaporized to mix with the air and ignite from the spark. Remember, the engine can burn only fuel vapor, not liquid fuel.
If everything goes according to plan, the engine fires up quickly on the richer fuel/air mixture, begins heating the internal engine parts and ultimately re-vaporizing the condensed fuel and burning it during the warm-up cycle. This is why an engine will flood so quickly in extremely cold weather if it doesn't start in the first few seconds of cranking.
In that scenario, the injectors continue to deliver fuel and more and more of it condenses in the combustion chamber, wetting and ultimately "drowning" the spark plug, making it incapable of firing. The result is a flooded engine.
In some cases, holding full throttle to "unflood" the engine by maximizing air flow and reducing fuel flow will allow the engine to start. When the engine management computer recognizes a wide-open throttle at cranking rpm, it cuts the injector pulse width in half to reduce fuel flow to help clear a flooded condition.
In your area if cold, it would seem that at least some liquid fuel generated on cold starts is ending up in the crankcase mixed with the oil. A small amount isn't harmful, and in most cases will evaporate during a fully warmed-up drive -- perhaps 20 minutes. The danger, of course, is that gasoline is an effective solvent, diluting the oil and reducing lubrication to sensitive engine parts.
You should carefully check the components and function of the positive crankcase ventilation (PCV) system to determine if it's functioning properly.
It should draw combustion gases and pressure out of the crankcase and through the induction system.
If the PCV valve or system is not working correctly, excess combustion blow-by, including raw fuel from a cold start, can end up in the crankcase. An iced-up PCV valve, leaking or plugged hose or filter can effectively disable the PCV system, resulting in excess crankcase pressure.
