How much oil does my air compressor need? It’s a straightforward question, and shouldn’t be a difficult question to answer. However, sometimes it is. This page is addressed at the DIY air compressor owner and the users of these, relatively low cost, home and small workshop air compressors.
Know that if you have an oil-lubricated compressor, you will need to add oil to the sump from time to time. The normal operation of the compressor pumps allows small amounts of lubricating oil to migrate from the sump, up the cylinder wall, and into the air compression chamber. From there the oil flows into the tank along with the compressed air.
Pumps with bad cylinder seals will allow even more oil to leave the compressor sump.
How Much Oil Does My Air Compressor Need? Not Too Much!
Overfilling the oil sump on a lubricated air compressor can create problems. Better to under fill the sump just a bit than overfill it.
The logical first step to find out how much oil the air compressor needs is to check the manual that came with the air compressor, if you still have it. If your air compressor is oil lubricated, the manual will indicate so, and tell you how much oil the sump needs to be full.
Of more use in checking the compressor oil, since who can find their compressor manual years after we bought the darn thing, is to use either the sight glass or the oil dip stick.
Compressor Oil Level Sight Glass
If your air compressor uses a sight glass to help determine the oil level in the sump, examine yours, and see where the oil level is. If the oil level is at the middle of the center dot (it’s a red one on this sight glass) then your oil is at the correct level. An oil level any higher than this would be considered over-filling.
No sight glass? Then if your air compressor is oil lubricated, it will almost invariably have a dipstick.
Compressor Oil Dipstick
The oil level dipstick is typically part of the oil-fill tube cap.
The oil fill cap inserts into the oil fill tube, typically found near the tip of the oil sump, as can be seen in the following image.
The dipstick and the cap have a small hole in them to allow the oil sump to breathe. Absence of the hole, or if the hole is plugged, may lead to pressure or vacuum to develop in the sump.
No Dipstick & No Sight Glass
If this is the case with your air compressor, you may want to double check to be sure that it actually is a compressor to which you must add lubricating oil. Many modern compressors are built to be self-lubricating, and no external lubrication is needed.
In the event your compressor is – for sure – oil lubed, does not have a sight glass, does not have a dipstick, you have a couple of options.
Take the measurements of your oil sump. Close counts. Take a photo of it from two sides. Take the photo and the dimensions with you to the compressor store. Find a compressor on display with a comparably sized oil sump, find the manual for the new compressor, and note the correct oil level for that compressor, and use that as a guide for yours.
Or, with a handy supply of rags (you will need them) and a large container, pull the sump drain plug (can be seen in the image above) and drain all the oil into the container. Measure the volume. Use an ounce or two more than this volume to refill the oil sump with fresh oil. Cross your fingers!