When an over-rich fuel mixture enters the cylinders, or when the engine is rarely or never required to develop its full power, or when the engine is often operated in a cold state (short city trips), resulting in incomplete combustion of the fuel. Carbon deposits, oil hydrocarbons and other products, unburned fuel itself and moisture condensates cause the formation of sludge, acid and asphalt. Unburned fuel falls on the cold walls of the cylinders and flows down them into the crankcase, while the oil film is washed off the walls of the cylinders and pistons. The consequence of this is the deterioration of the lubrication of the cylinder walls and the dilution of the oil, which affects the lubricating properties of the oil, depending on the amount of fuel that has entered the oil.
In case of severe dilution of the oil, it should be replaced ahead of schedule. Since with a sharp driving style (hot engine) gasoline particles evaporate in the oil, the oil level should be checked more often in winter (numerous cold starts - high proportion of gasoline in oil).