Dear friends

one of my customers complained that one of his walk-in coolers is abnormal , when checking oil in sight glass it looked in grey color , so i took a sample of oil, it seemed to be contaminated with bearing or connecting rod material , i disassembled the compressor and checked i found every thing is normal,i cleaned all parts then assembled again , charged new oil and operated again but after 2 hours of operation the oil color again turned into same color , after repeating oil replacement for 5 times and operating compressor for same period of time the situation didn't change , i noticed that liquid is flooding back to compressor , i appreciate you help to find the possible causes of this phenomena , thanks in advance
helpful information:
compressor type : Bitzer 18hp
refrigerant : R404A
oil type : POE