Use a non-detergent air compressor oil made for your type of compressor, in the ISO viscosity grade the manual specifies. Piston compressors take a reciprocating compressor oil (categories DAA or DAB). Oil-flooded screw and vane compressors take a rotary compressor oil (DAG, DAH or DAJ). Engine oil is not a substitute unless the compressor manual explicitly allows it.
This page explains how to match air compressor oil to the machine, when synthetic oil is worth it, and what to check before an oil change. It is published by Zhongtian, which makes compressor oils for both piston and screw compressors.
Which oil does each type of air compressor need?
The compressor design decides the oil category; the viscosity grade is a separate choice. ISO 6743-3 classifies compressor lubricants, and the categories below are the ones used for air compressors.
| Compressor type | How the oil works | Oil category | Zhongtian product |
|---|---|---|---|
| Reciprocating (piston) | Oil lubricates cylinders, rings and valves; valves run hot | DAA (light duty) or DAB (medium duty), specified in ISO 6521-1 | L-DAB air compressor oil |
| Drip-feed rotary vane | Oil is fed to the vanes in small amounts | DAA or DAB | L-DAB |
| Oil-flooded rotary screw or vane | Oil is injected into the compression chamber to seal, cool and lubricate, then separated from the air | DAG, DAH or DAJ, specified in ISO/TS 6521-2 | L-DAG (light duty); L-DAJ synthetic |
| Centrifugal | Oil lubricates bearings and gears only, not the air path | Usually a turbine-type oil named by the OEM | Ask us with the OEM specification |
| Oil-free (dry) screw or piston | No oil in the compression chamber | Only the gearbox or bearings are lubricated, to the OEM specification | Ask us with the OEM specification |
The most common mistake is buying by viscosity alone. ISO VG 46 is sold for both piston and screw machines, but a reciprocating oil such as L-DAB 46 is not formulated for the continuous oil-and-air contact inside a flooded screw compressor. Our L-DAB 46 guide explains the codes in more detail.
Can I use motor oil in an air compressor?
Not unless the compressor manual allows it. Engine oils carry detergent and dispersant additives designed for combustion engines. In a compressor those additives do not help, and they can leave deposits on hot valves. Some small piston compressor manuals do list a non-detergent SAE 30 oil; if yours does, that is the only engine-type oil to use, and it should say non-detergent on the label.
Never use engine oil in an oil-flooded screw compressor. These machines run the oil through a separator and long service intervals, and need an oil from the DAG, DAH or DAJ categories.
Mineral or synthetic air compressor oil?
Synthetic oil earns its price mainly in screw compressors that run for long hours. In a flooded screw compressor the oil is in constant contact with hot compressed air, so resistance to oxidation decides how long it lasts. Our synthetic L-DAJ is specified for a working life of 4,000 to 8,000 hours, and a rotary compressor operating range of -40 °C to 110 °C. A piston compressor that runs a few hours a day is usually well served by a mineral DAB oil.
- Check compatibility before switching. Some synthetic compressor fluids, especially polyalkylene glycol (PAG) types, do not mix with mineral oil. Drain, and flush if the compressor maker requires it.
- Check seals and paint. The compressor maker’s approval list is the safe route when moving from mineral to synthetic.
- Compare on hours, not price per litre. Fewer changes and fewer deposits can outweigh the higher purchase price in continuous duty.
What viscosity compressor oil do I need?
The ISO viscosity grade the compressor maker specifies. ISO VG numbers come from ISO 3448: ISO VG 46, for example, means a kinematic viscosity of about 46 mm²/s at 40 °C, within ±10%. Many rotary screw compressors use ISO VG 32 or 46, and piston compressors often use heavier grades, but the manual is the reference, adjusted where the maker allows for very cold or very hot sites.
Why does compressor oil turn to varnish and carbon?
Oxidation. Hot air and oil form acids, then varnish, then hard carbon on the hottest surfaces: valves and discharge ports in a piston compressor, the separator and element in a screw compressor. Carbon insulates, so the machine runs hotter and makes more carbon. The usual causes are the wrong oil category, a drain interval stretched too far, or a compressor running hotter than designed.
When should I change air compressor oil?
Follow the manufacturer’s interval, and change early if the oil shows any of these signs:
- It turns milky, which means water has emulsified in the oil.
- It darkens quickly or smells burnt.
- Varnish or sludge appears on the sight glass, separator or valves.
- The compressor runs hotter than usual or the separator pressure drop rises.
Oil analysis (viscosity, acid number, water) is the reliable way to extend intervals on larger screw compressors.
Zhongtian air compressor oils
We supply L-DAB oil for reciprocating compressors, L-DAG for light-duty oil-injected rotary compressors (discharge pressure below 0.8 MPa and discharge temperature below 100 °C, as listed on the product page), and synthetic L-DAJ for screw compressors in longer service. Typical data for the ISO VG 46 grades are compared in our L-DAB 46 guide. Products are made to order, so viscosity and pour point limits can be written into the purchase specification. Send us your compressor type, model and current oil and we will recommend the matching category and grade.
Frequently asked questions
Is all air compressor oil the same?
No. Piston compressors and oil-flooded screw compressors use different oil categories, even when the viscosity grade is the same.
Can I use hydraulic oil in an air compressor?
Only if the compressor manufacturer approves it. Hydraulic oils are formulated for a different duty, and a compressor oil is the safe default.
How often should I check the oil level?
Before each use on small piston compressors, and daily on industrial machines, through the sight glass or dipstick.
Sources
- ISO 6743-3:2003, Classification, Family D (Compressors)
- ISO 6521-1:2019, Categories DAA and DAB
- ISO/TS 6521-2:2021, Categories DAG, DAH and DAJ
- ISO 3448:1992, ISO viscosity classification
- Zhongtian product data sheets for L-DAB, L-DAG and L-DAJ compressor oils