Compressor lubricants: selection, compatibility and oil changes

What does compressor oil actually do?

In a reciprocating compressor, oil mainly lubricates the crankcase, bearings and cylinder walls. In an oil-injected rotary screw compressor the job is much larger. Oil is injected directly into the compression chamber in large volumes and does four things at once.

• Lubrication. It forms a film in the rolling-element bearings and, in many designs, between the rotor flanks where the male rotor drives the female rotor.
• Cooling. Most of the heat of compression goes into the oil, which is why a healthy oil-injected screw discharges at about 75–95 °C rather than the far higher temperature of dry compression. The oil then rejects that heat in the oil cooler.
• Sealing. The oil film closes the small clearances between rotors and housing. Thin or degraded oil lets air slip back, and free air delivery falls.
• Cleaning and protection. It carries wear particles and dirt to the oil filter, and its additives protect steel surfaces against rust and oxidation.

Because the same fluid also passes through the separator element, the thermostatic valve, the minimum-pressure valve and, on some machines, the inlet valve actuator, its properties affect the whole package, not only the airend.

Types of compressor lubricant

Compressor fluids differ by base stock and additive system. The table below summarises the common families in general terms. It is not a substitute for the manufacturer’s approval list for your model.

Many manufacturers sell their own branded fluid and set the service interval around it. Using a different fluid can be acceptable, but only where the base chemistry, viscosity, additive type and seal compatibility genuinely suit the machine, and where you accept that the change may affect any warranty conditions with the manufacturer.

Why viscosity grade alone is not enough

Most screw compressor oils are ISO VG 32, 46 or 68. Two oils with the same grade can still behave very differently. When we check a proposed replacement, we look at the full picture.

• Base chemistry – mineral, PAO, ester or PAG, which decides miscibility and seal compatibility.
• Viscosity index – how much the oil thins as it heats. In a 45 °C Hyderabad summer this matters.
• Oxidation stability – resistance to thickening, acid formation and varnish at operating temperature.
• Foam and air-release behaviour – poor air release leads to oil carry-over through the separator.
• Demulsibility – how well the oil sheds water, important in humid monsoon months and on machines that run cool.
• Additive package – anti-wear, anti-rust and anti-oxidant chemistry that may react with a previous fluid’s additives.

The test is simple: the oil must suit the machine, the temperature and the environment it actually runs in. A grade printed on a drum is only the starting point.

What happens when oils are mixed?

Topping up with whatever oil is in the store is one of the most common mistakes we see on site. Some oils are fully compatible. Others are not, and the result can show up in a few weeks.

• Additive reactions that form sludge, gel or deposits on the separator and in the oil cooler.
• Foaming, which pushes oil through the separator and into the air lines.
• Loss of demulsibility, so water stays in the oil and corrodes bearings.
• Seal swelling or shrinkage, giving shaft-seal leaks.
• Rapid darkening and varnish on the thermostatic valve and minimum-pressure valve, which then stick.

If the current fluid is unknown, treat it as unknown. Do not top up with a different chemistry. Drain, flush where needed and fill with one confirmed fluid.

Changing over to a different lubricant

A planned changeover, for example from mineral oil to a synthetic, needs more than a normal drain and fill. A typical method is:

1. Identify the machine fully: make, model, serial number, airend type and the fluid currently in it.
2. Confirm the new fluid is suitable for that model and compatible with its seals, hoses and paint.
3. Run the machine to warm the oil, then drain it hot from the separator tank, oil cooler and lowest points of the airend circuit.
4. Replace the oil filter and, where condition demands, the separator element.
5. Where the old oil was degraded or chemistry is changing, flush with the new fluid for a short run, then drain again.
6. Fill to the correct level with the new fluid, run, check temperatures, pressures and leaks, and recheck the level after it settles.
7. Label the machine with the fluid name and record the date and running hours.

The first change interval after a conversion is often shortened, because residual deposits loosen and load the new oil and filter.

Signs your compressor oil needs attention

Simple checks done during routine service tell a great deal: colour, smell, clarity, level, and whether the machine runs within its normal temperature range. Where the machine is critical or large, oil analysis gives a trend rather than a guess.

How often should compressor oil be changed?

There is no single number. The manufacturer’s interval for a given fluid assumes a clean, cool, well-ventilated room. Real Indian plant rooms are often hotter and dustier. The interval should come down when:

• Ambient temperature in the compressor room regularly exceeds about 40 °C.
• Dust, cement, fibre or chemical fumes reach the intake.
• The machine runs long periods unloaded or lightly loaded and stays cool, so water builds up.
• Discharge temperature is consistently near the top of the normal range.
• Oil analysis shows rising acidity, oxidation or wear metals.

On our own rental fleet working on hire, we change oil and service every 300 running hours because portable machines work in heat and dust round the clock. Stationary machines in clean rooms usually run much longer intervals, set by the fluid and the condition.

Oil analysis: when it is worth doing

Oil analysis is a laboratory test on a small sample taken from the running machine. It is most useful on larger or critical compressors, on machines with long change intervals, and when you suspect a problem. A useful report looks at viscosity change, acid number, oxidation, water content, wear metals such as iron and copper, and contaminants such as silicon from dust. The value comes from trends: one sample shows a snapshot, a series shows whether the fluid and machine are stable. Take samples hot, from the same point each time, into a clean bottle, and record running hours.

Lubricants for portable and reciprocating compressors

Portable diesel compressors use two separate oil systems: the engine oil, chosen to the engine maker’s specification, and the compressor fluid in the airend circuit. They must never be confused. Reciprocating compressors use oil in the crankcase, and on lubricated cylinders, a grade that resists carbon formation on the discharge valves. Carbon build-up on piston compressor valves is a real fire risk in discharge pipes, so the right oil and correct quantity matter.

How Reddy Compressors helps

From our base at Tarnaka, Secunderabad, our service team works on oil-injected screw, reciprocating and portable compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and many other makes. We are an independent multi-brand specialist, not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.

• We identify the right fluid from your machine’s make, model, serial number and nameplate, and confirm the fluid currently in it before quoting.
• We supply compressor lubricants, oil filters, separators and service kits matched to the machine, delivered across India.
• Our engineers carry out oil changes, planned changeovers and flushing on site across Telangana and Andhra Pradesh.
• We investigate oil carry-over, high temperature trips, milky oil and sludge, and fix the root cause rather than just topping up.
• We build oil changes into AMC and preventive maintenance plans.

What to send us

• Compressor make, model and serial number, with a nameplate photo.
• The oil currently in use (drum label or product name) and when it was last changed.
• Running hours and average loading, if known.
• Any symptoms: temperature trips, oil in the air, milky or dark oil.
• Compressor room conditions: ambient temperature, dust, ventilation.

Frequently asked questions

Can I use any ISO VG 46 oil in my screw compressor?

Not safely. Viscosity grade is only one property. A screw compressor fluid must also have suitable base chemistry, oxidation stability, air release, demulsibility and seal compatibility for your machine and temperature. Hydraulic or engine oils of the same grade can foam, form varnish or carry over through the separator. Use a fluid approved for your model, or one confirmed as equivalent after checking its full data, and never mix chemistries.

Why is my compressor oil turning milky?

Milky oil means water is emulsified in it. The usual cause is a machine that runs too cool, often because it is oversized and runs lightly loaded, or because the thermostatic valve is stuck open, so moisture from the intake air condenses inside. High humidity in the monsoon makes it worse. Check the thermostatic valve and duty cycle, change the oil and filter, and correct the cause, or bearings will corrode.

Is synthetic compressor oil worth the extra cost?

Often, yes, for screw compressors in hot and dusty conditions. Synthetic fluids such as PAO resist oxidation better, allow longer intervals and keep coolers and separators cleaner. The benefit depends on the duty, room temperature and how well the machine is maintained. For a lightly used reciprocating compressor, a good mineral oil may be adequate. Converting needs a proper drain, compatibility check and sometimes a flush.

Why is my compressor consuming so much oil?

High consumption in an oil-injected screw usually means oil is leaving with the air. Common causes are a failed or wrong separator element, a blocked scavenge line or orifice, overfilling, foaming from mixed or wrong oil, or running below the minimum pressure the separator needs. External leaks at hoses, the shaft seal and fittings are the other cause. We check separator differential pressure, the scavenge circuit and the oil type.

Can I mix two different compressor oils?

Only if both suppliers confirm compatibility, and even then it is best avoided. Mixing mineral oil with PAG, or two oils with different additive systems, can create sludge, foam and varnish that block filters and stick valves. If you do not know what oil is in the machine, do not guess. Drain it fully, change the filter and refill with one confirmed fluid, then label the machine.

What oil does a portable diesel compressor need?

Two oils. The diesel engine needs engine oil to the engine maker’s specification, changed with its oil filter. The airend circuit needs compressor fluid to the compressor maker’s specification, with its own oil filter and separator. They are not interchangeable. In hot, dusty site work intervals should be shorter; on our own hire fleet we service every 300 running hours.

Related pages

• Lubricant, oil analysis and fluid condition
• Compressor oil filters
• Air-oil separator elements
• Compressor service kits
• Rotary screw compressor service
• Find my compressor part