Air Compressor Oil Basics: What You Need to Know

Sep 21, 2021 by Brad Taylor


Lubricated air compressors require regular oil changes and preventive maintenance to ensure proper operation. Operators need to understand what kind of oil to use in the air compressor, how often to change the air compressor oil and oil filters, and how to recognize common problems with air compressor oil. This guide will answer your questions about air compressor oil.

About Oil-Lubricated Air Compressors

Most air compressors used in industrial compressed air systems are lubricated (also known as oil-flooded air compressors). This simply means that oil is used to lubricate moving parts in the air-end (the part of the compressor where the air is compressed).

Lubricated air compressors include:

  • Oil-flooded rotary screw air compressors
  • Oil-flooded rotary vane air compressors
  • Lubricated piston-style (reciprocating) air compressors

Of these, oil-flooded rotary screw air compressors are the most common type found in industrial applications such as pneumatic conveying and powering tools and manufacturing lines. Oil is injected into the rotor housing during compression. The oil performs several functions.

  • Lubrication: Oil lubricates the moving parts to reduce friction and help them move more freely.
  • Cooling: The oil helps to cool air during compression to prevent air compressor overheating.
  • Sealing: The oil helps to create a seal between rotors to trap air inside for compression.
  • Wear prevention: By reducing friction, the lubricant also prevents premature wear and tear on moving parts and extends the life of the air-end.
  • Cleaning: As the oil circulates, it also cleans the system by capturing nanoparticles from the compressor.

In an oil-flooded air compressor, oil mixes with air during compression. It is then separated back out before compressed air leaves the system.

Do All Air Compressors Require Oil?

There are some types of air compressors that do not require lubrication, known as oil-free air compressors. They are sometimes used for specialty applications requiring ultra-clean air with very low oil carryover (oil left in the compressed air supply)—for example, food processing, medical compressed air, pharmaceutical manufacturing, and laboratory applications. However, in most cases, it is possible to remove enough oil from the air supply through inline filtration to meet even highly demanding specifications.

Aire Tip: You may not need an oil-free compressor to get ultra-clean compressed air. Proper filtration can reduce oil carryover to nearly zero. Read more: Do You Need an Oil-Free Air Compressor? Probably Not.

What Kind of Oil Do You Need for an Industrial Air Compressor?

There are three basic types of air compressor oil: petroleum-based (mineral), synthetic, and food-grade. Many air compressors use a mineral blend or synthetic oil, such as synthetic hydrocarbon, polyol ester, or polyglycol. Before selecting a lubricant for your industrial air compressor, check your manual and warranty. Always use the lubricant recommended by your manufacturer for your specific make and model of air compressor.

Selecting the Right Air Compressor Oil

Make sure you use a lubricant stock appropriate for your air compressor and for the conditions under which it will operate. These are the most common lubricants used for air compressors.

  • Synthetic hydrocarbons (POAs) are the most common lubricants used for industrial air compressors. POAs are derived from ethylene (from natural gas or petroleum). They are engineered for high stability, low volatility, and uniform flow across a wide temperature range. This is a good multi-purpose air compressor oil.
  • Polyol esters (POEs) are the second most common air compressor lubricant. They are derived from alcohol and carboxylic acid rather than petroleum sources. They offer outstanding thermal stability, excellent longevity and mix readily with other additives and lubricant stocks. POEs also act as dispersants, which can help keep lubricated parts clean. However, they can be damaging to paints and finishes and may react with certain rubbers or plastics.
  • Polyglycols (PAGs) are another group of non-petroleum synthetic lubricants. Their chief advantage is that, unlike most air compressor oils, they do not oxidize to create “varnish” on the inside of the air compressor. (More on varnish below.) However, PAGs are miscible with water, meaning they will mix readily and are difficult to separate with a typical oil/water separator. This reduces their viscosity over time as more and more water becomes mixed with the lubricant, reducing their effectiveness. Mixing PAGs with POAs or mineral-based lubricants may cause damage to your air compressor.
  • Blended products: Many air compressors use a blend of POA and mineral oils. This is a more cost-effective alternative to a pure POA base. Blends are often used for applications that require frequent oil changes (for example, environments in which oil is contaminated quickly by particulates at the air inlet). Mineral oils in blended products are highly susceptible to thermal breakdown and oxidation when exposed to the high heat of an air compressor, resulting in faster development of varnish inside the compressor. When using mineral oils or blends, the oil must be checked frequently for degradation and development of varnish.
  • Food-grade oils: Food-grade air compressor oils are synthetic oils that are non-toxic and appropriate for use in applications where compressed air may come in contact with food, such as food packaging lines.

In addition to the base material, you also have to consider the lubricant weight or viscosity. Most air compressors take a 20-weight or 30-weight non-detergent oil. 20-weight is typically recommended for colder environments and 30-weight for warmer environments. Always use the recommended oil type and viscosity for your air compressor model, climate conditions, and usage patterns.

What Happens If I Use the Wrong Air Compressor Oil?

Using the wrong oil in your air compressor can cause several problems, including:

  • Poor lubrication, resulting in increased friction and heat.
  • Formation of varnish and deposits inside the air-end.
  • Excess wear and tear on moving parts and shortened equipment life.

Mixing incompatible lubricants together can also result in serious damage to your equipment. If you have added the wrong oil to your air compressor, it is best to flush it out of the system and replace it with the appropriate lubricant as soon as possible.

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How Often Should Oil Be Changed for an Industrial Air Compressor?

Lubricated and oil-flooded air compressors must have regular oil changes to stay in proper working condition. This should be done as part of your regular preventive maintenance (PM) service. Typically, mineral oils and blends must be changed every 4,000 operating hours. Synthetic oils may last up to twice as long. Oil should be changed at least every 6 to 12 months even if the service hours have not been reached. Oil may need to be changed more often if your air compressor is running in very dirty conditions. Your operating manual for your air compressor will provide information about the proper service intervals for your make and model.

How Do I Know if the Air Compressor Needs More Oil?

Check the oil level at least 3-4 times each week and add oil if levels are running low. Your air compressor will have either a dipstick or a sight glass to check the oil level. With a calibrated dipstick, make sure your oil level is always at or near the max. With a sight glass, aim for an oil level about 2/3 full on the glass. (Check your owner’s manual for details for your model.)

Be careful not to overfill the air compressor oil. Adding excess oil may result in oil coming out of the compressor lines or the air compressor “spitting” oil. Excess oil in the compressed air supply may overload air oil separators and inline filters and result in damage to pneumatic tools and equipment. If you have overfilled your air compressor oil sump, drain a bit until it comes down to the proper level.

If you are noticing oil levels going down significantly in between oil changes, you may have a leak, failed air-oil separator, and/or a clogged scavenger recovery line. Check for oil pooling around the base of the compressor, visually inspect the scavenger line and air oil separator for defects. If you suspect that you have an oil leak or oil carryover, call for service right away to ensure that your compressor does not become damaged.

How Do I Know if the Air Compressor Oil Needs to Be Changed?

Oil must be changed at least once a year, regardless of operating conditions and hours of operation. This is because oil will degrade over time and no longer provide proper lubrication. In most cases, the oil will need to be changed more frequently. If oil is dirty or contaminated, it should be flushed and replaced with fresh oil. Signs that your oil needs to be changed include:

  • Oil that is thick or opaque
  • Discoloration
  • Visible grainy particles
  • Bad smell

If you notice any of these signs, change your oil right away and flush the old oil out of the system. If you are seeing a lot of contamination in your oil, an intake filter may help to extend oil life.

Other Maintenance for Lubricated Air Compressors

In addition to checking and changing the oil, it is important to take care of the oil filter and the air/oil separator. You should also inspect your system regularly for signs of varnish buildup.

Compressed Air Oil Filters

Lubricated air compressors must have an oil filter. The oil filter removes excess particulate from oil as it is recirculated through the compressor. The oil filter should be changed or cleaned each time you perform a complete oil change on your system. Depending on your compressor model, you may have a simple strainer that filters out only coarse particulate, or a disposable filter with folded filter media inside, much like the oil filter for your car or lawnmower. The oil filter should have a bypass valve to ensure that oil will keep circulating even if the filter becomes clogged—because even dirty oil is better than no oil.

Oil/Water Separators

Liquid water (condensate) is produced as excess moisture is squeezed out of the air during compression. In oil-flooded or lubricated air compressors, this liquid water becomes mixed with oil. The oil/water separator collects liquids and separates the oil from the water. This allows clean water to be disposed of safely for compliance with environmental regulations. The oil/water separator should be checked weekly to monthly and changed quarterly to annually, depending on your usage patterns.

Aire Tip: Proper maintenance of your oil/water separator will help you remain in compliance with environmental regulations. Here’s what you should know: Oil/Water Separators for Compressed Air Systems—Complete Guidelines

Preventing Varnish in Compressed Air Systems

Varnish is a term to describe the by-products of lubricant degradation. As lubricants break down, they deteriorate into new substances that are dark, sticky, and thick. If allowed to accumulate, it will eventually harden on surfaces. This is varnish, and it causes a number of problems for your air compressor. Air compressor varnish:

  • reduces lubrication effectiveness and increases wear on moving parts;
  • gums up control valves and other compressor components;
  • plugs oil ports and strainers;
  • causes the compressor to run hot; and
  • attracts and holds onto other harmful contaminants, further gumming up the works.

Heat is one of the biggest factors in varnish formation. As the compressor becomes compromised and runs even hotter, more varnish is formed, creating a self-perpetuating cycle. That’s why it is very important to prevent varnish formation as much as possible and take care of the problem immediately if varnish does develop. To prevent and mitigate varnish, take these steps:

  • Use the right oil base for your compressor model, environmental conditions, and usage.
  • Prevent system overheating and take action if your system is running hot.
  • Perform a regular oil analysis to look for early signs that varnish is developing. A laboratory can perform these tests for you.

If your system shows signs of varnish, you may need to use a top-treat or run-in cleaner. These cleaners can be added with the oil change to remove varnish buildup from inside the compressor. Talk to your manufacturer or consult your owner’s manual before using a cleaning treatment to make sure it is appropriate for your system.

Reducing Oil Carryover

Oil carryover is the amount of oil in compressed air, measured in parts per million (PPM). The heat of compression causes a small amount of oil to be aerosolized and mixed with the air supply. To ensure a supply of clean, dry compressed air, use an inline filter to remove both oil and particulate from your compressed air supply. For oil-flooded air compressors, the best choice is a coalescing filter, which removes both aerosolized oil mists and dry particulate from compressed air.

Aire Tip: Change the inline filter at least annually or every 8,000 hours of operation. Learn more: How to Use and Maintain Inline Filters for Your Air Compressor

Taking Care of Air Compressor Oil

If you need help maintaining your air compressor, Fluid-Aire Dynamics can support you. Contact us if you have questions about the oil for your air compressor or suspect that you may have an oil leak, varnish, or other oil-related problem with your compressor.

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