A/C Compressor Oils: What They Are, What’s Out There And Saving The Environment

December 30, 2019

In any air conditioning system in which a compressor is present, compressor oil will be necessary to ensure it’s functioning properly. Any given compressor oil has three main functions – removal of heat, sealing the system, and lubrication. The compressor works to compress refrigerant in an A/C system, but without the right lubricant present, the system won’t work how it should and even risks failing. Depending on the type of refrigerant used, there are also different types of compressor oils that work best with each type. Earlier refrigerants were proven to be particularly troublesome with regard to ozone depletion. Now, there’s a much wider range of environmentally friendly refrigerants that require a compatible form of compressor oil.

Importance of A/C Compressor Oil

Compared to other oils such as those used in a car, compressor oil serves an integral part of keeping an A/C compressor functional. Compressor oil captures both moisture and dust from the air, which can both be filtered out by the oil filter as the oil moves through the system. Next, the oil works to seal all the gaps within the compressor, so that the compressor can actually do its job without having to work harder due to pushing in through the gaps. A compressor also produces a lot of heat. Thanks to the constant movement of the piston, proper compressor oil works to absorb that heat, which can be dealt with by the oil cooler relatively quickly. Finally, the compressor oil also works to reduce the amount of wear and tear any moving part suffers throughout its lifetime.

Add all three of these functions together, and it’s easy to understand why A/C compressor oil is a valuable component of any compressor system. Without it, the lifespan of a compressor is seriously reduced.

Properties of A/C Compressor Oils

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Chemical stability

Since there are many different types of refrigerants used in A/C systems, it’s important that the chemical stability of any given oil be provided, so only compatible oils and refrigerants are used. If the wrong type of oil is used, everything from sludge to carbon deposits can be formed, which will negatively impact the functioning of the compressor.


The viscosity of compressor oil matters when you consider its ability to properly seal a system. The oil that isn’t viscous enough will have difficulty creating a proper seal, while being too viscous means that it will have difficulty spreading to all portions of a system. In general, a larger compressor with larger gaps will require thicker oil than smaller compressors.

Dielectric strength

This property of oil refers to how good of an electrical insulator the oil is, and is important in ensuring that the oil doesn’t spread electricity throughout the entire system. Since the oil is most often in direct contact with a motor coil, high dielectric strength is preferred.

Moisture tolerance

Oils have a tendency to dissolve a certain amount of moisture and water. When this water/moisture comes in contact with the refrigerant, it risks freezing into ice which will expand, causing potential damage to the compressor. When water is absorbed into the oil, it also has a negative effect on the dielectric of the oil overall, so how much moisture oil will take on and at what rate is an important quality.

Types of Compressor Oils

There are currently three different types of compressor oils in use. The first one is mineral oil, while the other two are considered to be synthetic oils. While mineral oil is largely outdated now, it’s important to understand its place in the history of compressor oil, as well as the steps that were necessary to adapt after its removal.

Mineral oil

Up until the 1990s, mineral oil was widely used as the main A/C compressor oil since it worked well with R-12 refrigerant. However, over time, it was proven that R-12 had significant ozone-depleting qualities, which made it a lot less appealing in an increasingly ecologically conscious world. As time marched on, more environmentally conscious refrigerants were made, and R-12 was discontinued. Mineral oil was, too, as it didn’t mix as well with these new refrigerants.

Polyalkylene glycol oil

This type of oil, often shortened to PAG oil, mixed extremely well with the new R-134a refrigerant that quickly came to replace R-12. To this day, PAG 46 oil remains one of the most commonly used oils in compressors nationwide. The numbers refer to the viscosity of the oil, with PAG 100 being common, and PAG 150 making a rare appearance.

In today’s era, polyalkylene glycol oil is the most ubiquitous type of compressor oil out there since R-134a refrigerant is so commonly used now. It has now been determined that while R-134a has little in the way of ozone-damaging effects, there are better options in refrigerants that have a lower environmental footprint overall.  The new R-1234yf refrigerant is quickly making PAG obsolete, with most manufacturers making their own in-house oil formulation that works best with their A/C compressors.

Ester oils

One issue with PAG oils and mineral oils is that they can’t be mixed in any amount if the compressor is expected to work at full efficiency. Ester oils can be mixed into systems that have some residue of mineral oil remaining, so these oils were mainly used in retrofitted vehicles that needed to begin using R134a refrigerant, but couldn’t use PAG due to residual mineral oil.


Hopefully, this can serve as a guide as to what exactly A/C compressor oils are used for, how they’re graded, and of course, what the different options are out there. It’s important to note that PAG oil can’t be used in any system that previously used mineral oil. Even small residue is enough to seriously impair the functioning of the compressor. It’s obvious that the popularity of anyone given oil is tied very closely with what refrigerant is currently in use. As we move toward finding more refrigerants that have better environmental potential, we should expect the oils used with those refrigerants to adapt and change in kind.

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