Wrong viscosity, mixed brands, or oil that's been run far past its service life. Bad oil can cause foaming, poor separation, and oil carry-over.

What you'll see

Oil carry-over that started after an oil change (wrong oil or mixed oils). Or gradual increase in oil carry-over as old oil degrades. The oil might look milky or foamy (mixed oils or water contamination), black and thick (severely degraded), or have a burnt smell. Compressors that don't run regularly are especially prone to water in the oil -- they never warm up enough to evaporate the moisture.
Before you assume this is the problem

Verify the separator and scavenge line are OK first -- those are more common and easier to check. If the separator is new, scavenge is clear, and oil level is correct, then look at the oil itself. See: Separator Element Worn, Scavenge Line Blocked.

Could also be:

See all causes of oil carry-over →

How to diagnose

  1. Check what oil was used

    Verify the oil type against the manufacturer specification. Check the viscosity grade -- D46 for cooler environments (0-25 degrees C ambient), D68 for warmer conditions (25+ degrees C). Check if different brands or types were mixed during the last top-up or change. Even the 'correct' oil type from a different brand can cause problems when mixed with the existing oil.
    Result: Wrong type, wrong viscosity, or mixed oils = drain, flush, and refill.

  2. Inspect the oil condition

    Take a sample or look through the sight glass. Clear amber (mineral) or greenish (synthetic) = good. Black = severely degraded. Milky/white/foamy = water contamination or mixed oils. Thick and sticky = oxidized, way past service life. If the oil turned black within days of a fresh change, either the wrong oil was used or the system wasn't properly flushed of the old degraded oil.
    Result: Abnormal oil appearance = change immediately.

How to fix it

  1. Drain completely and flush

    Don't just top up or do a partial change. Drain all the old oil. If the oil was severely degraded or the wrong type was used, flush the system by filling with fresh correct oil, running for a short period (30-60 minutes), then draining again and refilling. Contamination lingers in the cooler, piping, and separator vessel.

  2. Refill with correct oil only

    Use the exact oil specified by the manufacturer. Match the viscosity grade to your ambient conditions. Don't substitute thicker oil to try to mask a separation problem -- it doesn't work and can cause other issues.

Common mistakes

Never mix compressor oil brands, even if both claim to be the same specification. The additives in different brands can react with each other and cause foaming. Also: don't use automotive engine oil, hydraulic oil, or transmission fluid in a compressor. I've seen someone put transmission fluid instead of PAR oil in a screw compressor -- even after draining and refilling correctly, the contamination lingered and caused problems for weeks.

Parts & tools

Correct compressor oil (type and viscosity per manufacturer). Drain pan. Fresh oil for flushing if contaminated.

Review safety precautions before starting →

This issue can also cause

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