Using the wrong oil in a screw compressor -- engine oil, hydraulic oil, or a non-compressor-specific oil. These oils lack the anti-foaming additives that screw compressor oil contains. The oil foams in the separator, overwhelming it, and passes into the compressed air.

What you'll see

Oil in the compressed air that started after an oil change. The compressor may also run slightly hotter than normal because the wrong oil may not have the correct viscosity or thermal properties. If engine oil was used (a common mistake because it is readily available on site), the oil will foam excessively during compression. The separator cannot handle the foam, and oil carry-over increases dramatically.
Before you assume this is the problem

If the oil has not been changed recently and the correct oil type was used, the problem is not the oil type. Check the separator, scavenge line, temperature, or oil level. See: Dirty Separator, Oil Level Too High.

See all causes of oil in compressed air →

How to diagnose

  1. Verify the oil that was used

    Ask who did the last oil change and what oil they used. Check the oil container if available. Screw compressor oil is specifically formulated with anti-foaming agents, oxidation stability, and water separation properties. Engine oil (15W40 etc.) has detergent additives that actually promote foaming in a screw compressor -- the opposite of what you need.
    Result: Wrong oil type identified = drain and refill with correct oil.

How to fix it

  1. Drain and refill with correct compressor oil

    Drain all the oil from the system. Run the compressor briefly with fresh correct oil and drain again to flush residual wrong oil from the cooler, piping, and element. Refill with the correct compressor oil to the proper level. Also replace the separator element, as the wrong oil will have contaminated it. Replace the oil filter as well.

  2. Label the machine clearly

    Put a label on the oil filler cap specifying the correct oil type and quantity. This prevents future mistakes by operators who may not be familiar with the machine. Common compressor oils: Shell Corena S3 R or S4 R, Mobil Rarus SHC 1000, Atlas Copco Roto-Inject, or equivalent.

Common mistakes

The most common mistake is using engine oil because it is what is available on a construction site. It works temporarily but causes excessive oil carry-over and accelerated separator fouling. Also: do not mix oil types. If you do not know what oil is in the machine, drain it all and start fresh with the correct type. Mixing mineral and synthetic compressor oils can cause compatibility issues.

Parts & tools

Correct screw compressor oil (full system volume plus extra for flushing). New separator element. New oil filter. Container for draining old oil. Labels for the filler cap.

Review safety precautions before starting →

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