1. Oil level too low. 2. The oil suction tube is not immersed in engine oil and air enters the tube. 3. Oil filter paper gasket is not installed correctly or is torn. 4. The oil filter check valve spring is damaged. 5. Worn oil pump gears. 6. The support bearing fit gap is too large.
1. Top up. 2. Reassemble and check that the oil pickup is intact. 3. Reassemble or replace it. 4. Replace it. 5. Replace the pair or install a thin paper gasket. 6. Check and replace them if necessary.