Oil in the air compressor's air filter is usually caused by an abnormality in the oil-gas separation system or a component malfunction. The following steps can be used to troubleshoot and resolve the issue:

Check Oil Level:After stopping the machine for 10 minutes, check the oil level indicator. If the oil level exceeds the maximum mark (MAX line), drain the excess oil through the drain valve to restore the oil level to near the midline. 

An excessively high oil level will increase the pressure inside the oil-gas separator, causing oil to enter the intake side with the airflow.

Check Oil Separator Element: Remove the oil separator element cover and check if the sealing gasket is intact and correctly installed (no missing parts, reversed installation, or aging/deformation), ensuring good sealing performance.

Verify that the oil separator element model matches the air compressor. An incompatible model may lead to incomplete oil-gas separation.**

Check that the oil separator element fixing screws are tightened and that there are no air leaks at the interfaces.** If the oil separator element is damaged or has a poor seal, replace it immediately.

Check Return Oil Pipe:Use low-pressure compressed air to blow through the return oil pipe to check for blockages. If blocked, clean out impurities or replace the return oil pipe.

Check Return Oil Pipe: Blow through the return oil pipe with low-pressure compressed air to check for blockages. If blocked, clean out impurities or replace the return oil pipe. Check the return oil pipe connection for tightness, ensuring it is not loose or misaligned. 

Connection problems can cause oil leakage into the intake system, requiring reconnection or adjustment.

Check the intake valve. Inspect the intake valve sealing surface for cleanliness, damage, or sticking. If the sealing surface is dirty or damaged, clean or replace the intake valve to ensure a proper seal.

Check the cooling system. Clean the oil cooler fins to remove oil, dust, and other impurities to ensure effective heat dissipation. Poor cooling can lead to increased exhaust temperature, affecting oil-air separation.

Check the cooling water flow rate (for water-cooled models). Insufficient or excessive flow can affect exhaust temperature; adjust to the appropriate range.

Check the after-treatment equipment. Check the precision filter for blockage or if it has exceeded its expiration date; replace the filter element promptly. A malfunctioning after-treatment equipment may cause oil-laden air to enter the air filter without proper filtration.

If the problem persists after the above checks, it may be due to wear or malfunction of internal air compressor components (such as the rotor and bearings). It is recommended to contact a professional repair technician for further inspection and repair. After processing, the air filter needs to be replaced to avoid residual oil on the old filter affecting the filtration effect.