1. The combustion effect is not good, and the unfinished combustion products flow into the crankcase along the cylinder wall;
2. When the clearance between piston and cylinder wall is too large and the seal is not good, the mixed gas is jacked into the crankcase;
3. Fuel or coolant penetrates into the crankcase to dilute oil;
4, the anti-shear stability of the oil is not good;
5. The viscosity index is low, and the oil viscosity becomes smaller at high temperature.
