"Which oil should I put in my compressor?" The answer depends first on the technology: a screw compressor and a piston compressor do not use the same oil, the same oil change rhythm or the same checks. This article answers machine by machine.
One rule above all: your compressor manual always prevails. The values below are common trade practice, given as a guide to help you read a manual or an oil can label.
The essentials in one table
| Screw compressor | Piston compressor | |
|---|---|---|
| Role of the oil | Sealing, lubricating and cooling the compression stage | Lubricating the crankshaft, connecting rods and cylinders |
| Oil type | Dedicated screw compressor oil | Dedicated piston compressor oil |
| Most common grade | ISO VG 46 | ISO VG 100 |
| Cold room | Thinner grade (often ISO VG 32) if the manual allows | Thinner grade (often ISO VG 68) if the manual allows |
| Hot room | Thicker grade (often ISO VG 68) if the manual allows | Thicker grade (often ISO VG 150) if the manual allows |
| Oil change | Based on running hours, with oil filter and separator check | Based on running hours, and at least once a year |
| Level check | Sight glass on the separator tank, machine stopped and depressurised | Sight glass or dipstick on the crankcase, machine stopped |
Which oil for a screw compressor?
What the oil does in a screw compressor
In an oil-injected screw compressor, oil is injected continuously between the two rotors. It seals the gap between the screws so that air does not flow back, lubricates them, and absorbs the heat of compression before passing through the cooler. It is then separated from the air by the air/oil separator and returns to the circuit.
So it works hot, churned with air and moisture, all day long. That is why you cannot put just any oil in a screw compressor.
The right choice
- A dedicated screw compressor oil, formulated to resist oxidation, separate easily from air and water, and not foam.
- An ISO VG 46 grade in the vast majority of cases, for a temperate room.
- Synthetic if the machine runs many hours a day or the room is hot: the oil lasts longer and oil changes are further apart.
- An NSF H1 food-grade oil if the air can come into contact with food, drinks or food packaging.
Maintenance routine
- Check the level on the separator tank sight glass, with the machine stopped and depressurised for a few minutes. The level should read between the marks shown on the machine.
- Top up with the same oil, never with another brand or type.
- Change the oil at the intervals given in the manual, and always replace the oil filter at the same time.
- Keep an eye on the air/oil separator: rising oil consumption or oil in the network means it needs replacing.
The intervals are detailed in our article When should you service a screw compressor?.
Warning signs
- Milky oil: water has mixed with the oil. The machine is not warming up enough (too many short starts) or the room is very humid.
- Dark oil that smells burnt: overheating or an overdue oil change. Also check the cooler and the room ventilation.
- Level dropping quickly: a leak in the circuit or a saturated separator.
Which oil for a piston compressor?
What the oil does in a piston compressor
In a lubricated piston compressor, the oil stays in the crankcase. It lubricates the crankshaft, connecting rods and cylinder walls, much like in an engine, but without circulating in the compressed air, except in small amounts around the piston rings.
The right choice
- A dedicated piston compressor oil. Never engine oil: its detergent additives encourage deposits and emulsion with condensate water.
- An ISO VG 100 grade in most cases. Some manuals express it as an SAE grade (often SAE 30 or SAE 40): read the stated equivalence carefully.
- Mineral or synthetic according to the manual. Synthetic is worthwhile if the compressor runs for long periods or gets very hot.
Maintenance routine
- Check the level every week on the crankcase sight glass or dipstick, with the machine stopped and standing level. The level should be in the middle of the sight glass.
- Top up with the same oil, without going above the maximum mark: excess oil is thrown into the cylinders and ends up in the air.
- Change the oil at the intervals given, and at least once a year even if the compressor is used little: oil ages even when the machine is idle.
- Drain the tank regularly. Less water in the circuit also means oil that stays clean longer.
The details are in When should you service a piston compressor?.
Warning signs
- Oil in the outlet air: worn piston rings or the level is too high.
- Milky oil: condensate water in the crankcase, often on a compressor that runs in very short cycles.
- Rising oil consumption: wear on the piston rings or cylinders, to be checked.
Mineral or synthetic?
- Mineral oil costs less but oxidises faster, so oil changes are more frequent.
- Synthetic oil withstands heat better and lasts longer between changes. On a screw compressor that runs many hours or in a hot room, it often pays for itself through less frequent changes.
In both cases, the change interval follows what the manual specifies for the oil used. If the air can touch food, drinks or food packaging, choose a food-grade NSF H1 oil; for completely oil-free air, the answer is an oil-free compressor or enhanced filtration.
Costly mistakes, on both machines
- Mixing two oils. If you change to a different oil, drain completely.
- Using engine oil "to get by". It does not protect in the same way.
- Putting screw compressor oil in a piston compressor, or vice versa. The grades and formulations are not the same.
- Putting off the oil change. Oxidised oil turns acidic, clogs the filters and raises the temperature.
- Disposing of used oil carelessly. It is hazardous waste: take it to a recycling centre or an approved collector.
What about your AIRSTAHL compressor?
The oil reference, grade and quantity are given in the manual supplied with every machine. For screw compressors, the service kits bring together the oil and filters that match your model. Any doubt? Send us the model and serial number of your compressor and we will tell you the right oil.