Oil is the cheapest wear part in a compressor, yet it protects everything else. The wrong oil will not break the machine overnight: it wears it slowly, makes it run hot, clogs the filters and ends up costing a repair. The first rule is simple: the compressor manual always prevails. What follows helps you understand what it says, and why.
Why piston and screw compressors use different oils
In a piston compressor, the oil lubricates the crankshaft, connecting rods and cylinders, much like in an engine. It stays in the crankcase and only touches the compressed air in small amounts, around the piston rings.
In an oil-injected screw compressor, the oil is injected directly into the compression stage. It does three jobs at once: it seals the gap between the two rotors, lubricates them and, above all, removes the heat produced by compression. It circulates constantly, passes through a cooler, then is separated from the air by the air/oil separator.
Screw compressor oil therefore works much harder: it is churned with hot air all the time, which oxidises it. It must resist oxidation, separate easily from air and water, and not foam. These are not the same requirements as for a piston crankcase.
The three criteria that matter
1. Oil type: compressor-specific
Use a compressor oil, never engine oil. Engine oils contain detergent additives designed for combustion engines: in a compressor, they encourage deposits and emulsion with condensate water.
- For a piston compressor: a piston compressor oil, mineral or synthetic as the manual specifies.
- For a screw compressor: a screw compressor oil, formulated against oxidation and for good air/oil separation.
2. Viscosity
Viscosity is expressed as an ISO VG grade (the number gives the viscosity at 40 °C). As a general rule, piston compressors use a thicker oil than screw compressors, which need a thinner oil to circulate quickly through the cooling circuit.
Room temperature matters too: some manuals specify a different grade for a very cold or very hot room. Stick to the stated viscosity: oil that is too thick circulates poorly at start-up, oil that is too thin protects less well when hot.
3. Mineral or synthetic
- Mineral oil is cheaper but oxidises faster, so oil changes are more frequent.
- Synthetic oil withstands heat better and lasts longer between changes. It is especially worthwhile on a screw compressor that runs a lot or sits in a hot room.
Over time, synthetic oil often pays for itself through fewer oil changes. But the change interval must always follow what the manual specifies for the oil used.
Food and pharmaceutical applications
If compressed air can come into contact, even accidentally, with food, drinks or food packaging, use a food-grade oil (NSF H1 registered). It is a hygiene matter and often a quality-audit requirement. If you need air that is completely oil-free, the answer is different: enhanced filtration, or an oil-free compressor.
Mistakes to avoid
- Mixing two oils of different types or brands: the additives can react, forming deposits or foam. To switch oil, drain completely and flush if the manual says so.
- Topping up with engine oil "just this once".
- Exceeding oil change intervals: oxidised oil turns acidic, clogs the separator and raises the temperature.
- Forgetting the filters: on a screw compressor, the oil filter is changed at every oil change.
- Ignoring oil that changes appearance: milky oil means water (humid room, machine not running long enough to warm up); very dark oil means overheating or an overdue change.
Which oil goes in your AIRSTAHL
The oil type, viscosity and quantity are given in the manual supplied with every AIRSTAHL compressor. For screw compressors, the maintenance kits bring together the oil and filters that match your model. If in doubt, send us the model and serial number and we will give you the right reference.