Compressed air is often called the fourth utility in a factory. When a compressor stops, everything from CNC chucks to packing lines stops with it. Many compressor breakdowns trace back to oil: the wrong type, the wrong grade or oil left in too long.
Reciprocating (piston) compressors usually need a compressor oil of ISO VG 68 to 150 with low carbon-forming tendency (DIN 51506 VDL type). Oil-flooded rotary screw compressors usually need ISO VG 32 to 68, most often VG 46, with high oxidation stability and quick air release. Follow the compressor maker's grade, and never use engine oil.
The oil lubricates the crankcase and cylinders. Some of it is carried with the air past hot discharge valves. There, oil that is not designed for compressors can crack into carbon deposits. Carbon makes valves stick and leak, raises temperatures further and, in the worst case, can lead to fires in discharge lines.
What the oil needs: the right viscosity for cylinder and bearing lubrication, very low carbon-forming tendency and good oxidation stability. DIN 51506 VDL is the common specification.
Here oil is injected into the screw element. It cools the air during compression, seals the gaps between the rotors and lubricates the bearings. It then has to separate from the air in the oil separator and go round again, thousands of times a day.
What the oil needs: excellent oxidation stability (it is constantly mixed with hot air), quick air release, low foaming, good water separation and low volatility so that carry-over stays low.
| Reciprocating | Rotary screw (oil-flooded) | |
|---|---|---|
| Typical grade | ISO VG 68–150 | ISO VG 32–68 (46 most common) |
| Main risk | Carbon on discharge valves | Oxidation, varnish, separator blockage |
| Key oil property | Low carbon-forming tendency | Oxidation stability, air release |
| Typical change interval | Per manual, often 500–2,000 hours depending on size | Often 2,000–4,000 hours mineral; longer for synthetics |
| Signs of trouble | Sticking valves, high discharge temperature | Rising separator pressure drop, dark oil, high temperature trips |
Intervals vary widely with the oil, temperature and air quality, so use the manual and, for large compressors, oil analysis.
Compressing humid air produces water. In June to September, a compressor can condense several litres of water a day.
PAO and ester-based synthetic compressor oils resist oxidation much better and can extend intervals a lot in screw compressors. They cost more per litre but often less per running hour. Check seal and paint compatibility, and flush when changing from mineral oil.
Can I use hydraulic oil in an air compressor?Generally no. Some hydraulic oils share a viscosity grade, but compressor oils are designed for oxidation and carbon resistance at high temperatures. Use a compressor oil unless the maker approves otherwise.
Which oil grade is used in screw compressors?Most oil-flooded screw compressors use ISO VG 46. Some use VG 32 or 68 depending on design and climate. The manual is the final word.
Why does my compressor oil turn milky?Water. Humid air condenses in the oil, often because the compressor runs too cool or drains are not working. Change the oil and fix the cause.
How often should compressor oil be changed?It depends on the type and oil. Reciprocating compressors often need changes every 500–2,000 hours, depending on size; screw compressors 2,000–4,000 hours on mineral oil and longer on synthetics. Follow the manual.
Tell us whether your compressor is screw or reciprocating, its make and the oil grade in the manual, and our team will recommend the matching compressor oil.