The easiest cause to check and fix. When the crankcase is overfilled, the crankshaft and connecting rod dip into the excess oil and splash it up into the cylinder. From there it gets past the rings and into the compressed air. Takes two minutes to diagnose.
What you'll see
If the oil level is at or below the maximum mark, overfilling isn't the cause. Check ring wear, wrong oil type, or running temperature instead. See: Worn Piston Rings, Wrong Oil Type, High Running Temperature.
How to diagnose
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Check the oil level
With the compressor stopped and on a level surface, check the sight glass or dipstick. The level must be between minimum and maximum marks. Any level above maximum is overfilled. On compressors without clear marks, the manual specifies the correct fill quantity.Result: At or below max = not overfilled, look elsewhere. Above max = overfilled.
How to fix it
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Drain to the correct level
Open the drain plug and drain oil slowly until the level drops to the midpoint of the sight glass. Don't rush and drain too much. Replace the drain plug and check again. Run the compressor for a minute, stop, wait for oil to settle, and verify the level is correct.
People check the oil level while the compressor is running and see a low reading, then overfill. The oil gets distributed throughout the pump during operation. Always check with the compressor stopped for a few minutes so the oil settles. Also: when doing an oil change, don't just fill to the top of the sight glass. The maximum mark exists for a reason -- fill to it, not above it.
Oil drain container. Clean rags. Correct compressor oil if you drain too much.
Drain oil when stopped but warm. Hot oil can burn. Use a suitable container.