1. The wrong oil is being used. 2. The piston rings are worn out. 3. The piston rings are stuck, the oil return hole in the piston ring groove is clogged. 4. Leakage from under the rear oil seal. 5. Oil level too high.
1. Use oil according to instructions. 2. Replace them with new ones. 3. Remove carbon deposits and clean the piston. 4. Check, replace with a new oil seal and its cover. 5. Drain off the required amount.