What causes gas to mix with the oil in a 2.4 liter Nissan engine?

What causes gas to mix with the oil in a 2.4 liter Nissan engine?
The only way for gasoline to mix with oil in most engines is for one or more cylinders to be running EXTREMELY rich. Generally speaking, this is caused by an injector that is leaking badly due to either built up carbon on the injector nozzle or a damaged injector. This causes fuel to leak into the cylinder any time the fuel system is pressurized.
The excess fuel washes down the cylinder walls, stripping away the protective layer of oil and contaminating the oil in the crank case. Because the oil is being washed off of the cylinder walls, this leads to reduced engine life. Because fuel is being injected into the cylinder all the time, you will get reduced fuel economy.