How to Check and Add Oil to a Reciprocating Compressor

Oil does three big things inside a reciprocating compressor pump: it reduces friction, carries heat away from moving parts, and helps prevent premature wear. Running low can lead to overheating and early pump failure.  

What you’ll need 

• Compressor-approved oil (use the type and viscosity specified in your owner’s manual)
• Clean rags (wipe the area before opening the fill point)
• Funnel (helps prevent spills into the belt guard or motor area) 

Good practice: Keep a dedicated funnel for compressor oil only. Cross-contamination from other oils can cause foaming or poor lubrication.

1. Power OFF the compressor. Stop it using normal controls, then switch off and follow your site’s lockout/tagout practice if applicable. 
2. Drain all pressure from the unit. Depressurize the tank and lines before opening the oil fill. 
3. Let hot parts cool. Pumps and discharge lines can get hot enough to burn skin.
4. Work clean. Dirt in the oil fill is dirt in the pump.

• Drain all pressure from the unit. 
• Turn power OFF. 

Why: Oil readings can be misleading if the machine is running or pressurized and opening the fill point with pressure in the system is a safety risk.  

Most reciprocating compressor pumps have one of these: 

• Sight glass (a round “window” on the crankcase)
• Dipstick (often attached to the fill cap)

Locate the sight glass or dipstick before you start adding anything.  

•  Sight glass: Oil should be around the midpoint of the glass. 
• Dipstick: Wipe it, reinsert it fully, then read the mark per the manual.

If there is no sight glass: 

• Drain all pressure from the unit, then verify oil level by the fill port method described for that unit: filled to the top of the fill port threads. Also refer to the owner’s manual for the specific oil guidance.

Tip: If the oil looks milky, foamy, or smells burnt, treat that as a warning sign. In many shops, that triggers an oil change and a quick check for root causes (moisture, wrong oil, overheating). 

• Use compressor-approved oil and match the manual’s viscosity and type.

Why: Automotive oils may contain detergents and additives that can foam under compressor conditions.  

• Wipe dust and grime off the fill cap and surrounding crankcase. This keeps debris out of the pump.

3. Open the fill point

• Drain all pressure from the unit (again, if you have not already).
• Remove the fill cap.

4. Add oil slowly

• Use a funnel.
• Add a small amount at a time.  
• Pause and recheck the level.

 5. Stop at the correct level

• Do not overfill. Overfilling can cause oil carryover into the air system, mess up downstream equipment, and increase deposits on valves.  

 6. Recheck and close up

• Recheck the level after a minute (oil needs a moment to settle).  
• Reinstall the fill cap securely.
• Wipe any spilled oil.

 7. Restart and observe

Power the unit back on and watch for:

• Leaks around the fill cap or crankcase
• Abnormal noise
• Oil mist or heavy oil smell at the breather