Air Compressor Oil Basics: What Operators Need to Know

Lubricated air compressors require regular oil changes and 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 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 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. 

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

Other important components that are utilized after the compression process and outside of the air compressor itself include: 

• An oil/water separator is used to separate oils and lubricants from liquid water created during compression;
• Inline compressed air filtration is used to remove any last traces of oil carryover from the compressed air. 

Why Does My Air Compressor Need Oil?

Air compressor lubricants perform several functions. 

Lubrication

Compressor oil lubricates the moving parts to reduce friction and help them move more freely. 

Cooling

Air compressor oils help to cool air during compression to prevent air compressor overheating. 

Sealing

In rotary screw compressors, the compressor 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 compressor oil circulates, it also cleans the system by capturing nanoparticles from the compressor. 

Noise Reduction 

Proper lubrication helps to minimize noise generated by the compressor's moving parts, contributing to a quieter working environment.

Corrosion Protection 

Air compressor oil contains rust and corrosion inhibitors, which protect the compressor's metal components from damage caused by moisture and other contaminants. 

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 ultraclean 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 Is Used in an Industrial Air Compressor?

There are three basic air compressor oil types: petroleum-based (mineral), synthetic and food-grade. Many air compressors use a mineral blend or a synthetic oil, such as synthetic hydrocarbon, polyol ester or polyglycol. It is important to use a lubricant specifically rated for air compressors; do not use a general-purpose motor oil used for automotive or other purposes. These are the most common lubricants used for air compressors. 

Synthetic Hydrocarbons (POAs)

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)

The second most common air compressor lubricant. They are derived from an 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)

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, 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.

Standard vs. Synthetic Air Compressor Oil: Which Is Better?

Both standard air compressor oil (i.e., petroleum or mineral oil)  and synthetic oils are available for industrial air compressors. Here are some of the main differences. 

• Standard, or mineral, oil is generally less expensive than synthetic oil, making it a more cost-effective choice for day-to-day operation. It provides adequate lubrication and protection for most standard operating conditions. However, standard oil requires more frequent oil changes compared to synthetic oil, and it may not perform as well in extreme temperatures or under heavy loads. Standard air compressor oil is well-suited for less demanding or intermittent applications, and it works well in environments with moderate temperatures. If operating costs are a significant concern, standard oil can be a good option.
• Synthetic oil offers better lubrication and protection across a wider range of temperatures. This can help to reduce wear and extend the life of equipment, especially for rotary screw compressors.  It has higher resistance to oxidation, which leads to longer oil change intervals. Synthetic oil may also improve compressor efficiency and reduce energy consumption. Although it is generally more expensive than mineral oil, the benefits it provides can outweigh the costs in certain situations. Synthetic oil is ideal for continuous or heavy-duty applications and for environments with extreme temperatures (both high and low). It is also a good choice when longer maintenance intervals and reduced wear are desired, as well as in applications where energy efficiency is a priority.

Ultimately, the choice between standard and synthetic oil will depend on the specific requirements of your application, operating environment, and budget. If your compressor operates under demanding conditions or you require longer maintenance intervals and improved performance, synthetic oil might be the better choice. However, if your compressor runs intermittently or in moderate conditions, and cost is a primary concern, standard oil should be sufficient.

Choosing the Best Air Compressor Oil

 

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. Make sure you use a lubricant stock appropriate for your air compressor and for the conditions under which it will operate. 

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. When choosing between air compressor oil types, always use the recommended oil type and viscosity for your air compressor model, climate conditions and usage patterns.

Different types of oil have different properties that impact their overall performance and best uses. Some of these properties are intrinsic to the type of oil (e.g., mineral vs. synthetic compressor oils). Oils may also have additives that improve performance on various characteristics. Some of these additives include the following. 

Anti-Wear Agents

These additives help reduce friction and wear on the compressor's moving parts, thereby extending the service life of the components.

Antioxidants

Oxidation can lead to the formation of sludge, varnish, and other harmful deposits. Antioxidants help to slow down the oxidation process, extending the life of the oil and improving compressor performance.

Rust and Corrosion Inhibitors

These additives protect the compressor's metal components from rust and corrosion, ensuring their longevity and proper function.

Foam Inhibitors

Foaming can reduce the oil's ability to lubricate and cool the compressor's components effectively. Foam inhibitors help to minimize foaming, ensuring efficient heat transfer and lubrication.

Demulsifiers

Moisture can accumulate in the compressor system, leading to oil-water emulsions. Demulsifiers help to separate water from the oil, making it easier to remove the water and maintain the oil's lubricating properties.

Viscosity Index Improvers

These additives help maintain the oil's viscosity over a wide temperature range, ensuring consistent lubrication and protection under varying operating conditions.

Pour Point Depressant

These additives lower the oil's pour point, allowing it to remain fluid and pumpable at low temperatures, which helps ensure proper lubrication during cold starts.

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. 

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 of 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, and 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, 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 OIl-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.

Compressor Oil Sampling: When and How Often to Do It

Oil sampling is the process of collecting a small amount of compressor oil from an air compressor for analysis to assess the oil's condition, detect potential issues, and determine whether the oil needs to be changed or the compressor requires maintenance. It is an essential part of a proactive maintenance program, helping to prevent unexpected downtime and extend the compressor's service life.

The frequency of oil sampling depends on several factors, including the type of compressor, its operating conditions, the oil type, and the manufacturer's recommendations. As a general rule of thumb, compressor oil should be sampled every six months or after 2,000 hours of operation for rotary screw compressors. 

However, these intervals are only general guidelines, and it is crucial to consult your compressor manufacturer's recommendations and consider your specific application and operating conditions when determining the appropriate oil sampling frequency.

Let Us Take Care of Your Air Compressor Oil

Fluid-Aire Dynamics can help you with air compressor maintenance, including choosing the best air compressor oil, changing the compressor oil and oil filter, and oil sampling. 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.