The cylinder liner is leaking, the cooler is damaged, etc.
1. The cylinder liner is leaking. The cylinder liner seal ring is not good and leaking.
2. The cooler is damaged. The oil cooler is damaged and some of the cooling water leaks into the lubrication system.
3. The water pump is getting old. The hydraulic seal and mechanical seal of the cooling water pump are aged or damaged, and cooling water enters the lubrication system.