AMSOIL Compressor Oils Comparison Tests

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A Study of Compressor Oils
The most comprehensive Compressor oil testing
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Compressed air is a critical part of many manufacturing facilities; without it, production would cease. Reliable air compressor operation is essential to manufacturing production. Lubrication is key to keeping air compressors running and is sometimes called the compressor’s “life-blood.” Compressor lubricants are produced by many lubricant manufacturers, ranging in quality from poor to excellent. Poor air compressor oil could cause the compressor to have a very short life, but excellent quality air compressor oil reduces maintenance and can extend compressor life. Some end users find comfort in using the manufacturer recommended oil because of perceived quality.

The American Petroleum Institute (API) governs the minimum quality standards for engine oils. Air compressor oils are not governed by any organization, so no official performance standards exist. This leaves the responsibility for producing a satisfactory product to the individual lubricant manufacturers.

Air compressor original equipment manufacturers (OEM) help eliminate some confusion by publishing minimum oil specifications required for their individual air compressors. These minimum oil specifications insure minimum lubricant performance. Any oil that meets or exceeds the minimum specifications can be used without voiding the standard or regular compressor warranty (usually one or two years).While OEMs do not manufacture their own compressor oil, they frequently market their own brand of compressor oil and have often been able to tie separately purchased extended warranty requirements to the use of their own branded oil.

Air compressor companies must contract with a lubricant company to manufacture oil for them.When the air compressor manufacturers purchase oil from a lubricant manufacturer they become the middlemen and the cost is increased to the consumer. OEM branded compressor oils provide the allure of quality, but in many cases the price of these oils is unnecessarily inflated and is frequently exorbitant. In addition, these oils often do not have the best performance characteristics that are available on the market.


The purpose of this paper is to inform consumers about the performance and cost differences between compressor oils, including OEM branded and popular aftermarket products. These evaluations will provide consumers the confidence in which to purchase high-quality compressor oil at the lowest possible price.


The testing by which these oils are evaluated is done in accordance with American Society for Testing and Materials (ASTM) procedures. The results of these tests can be duplicated and verified by laboratories that conduct these ASTM tests. A notarized affidavit certifying the results are correct is included in the appendix. Compressor oil pricing was determined by contacting the manufacturer or the distributor and requesting a quote on a five-gallon pail and a fifty-five gallon drum of their lubricant. Price quotes obtained are listed in the pricing section.


This paper is focused on rotary screw type compressor oils. Rotary screw compressors are widely used in all types of industries, and synthetic compressor oils are recognized as superior to mineral-based compressor oils by compressor manufacturers as well as most oil companies and consumers. ASTM tests conducted were chosen respective of rotary screw compressor environments. While there are many tests by which oils could be measured, the information provided by the tests and included in this paper gives a well-rounded view of the compressor oil’s performance.

Desired Performance Characteristics and Results

Oils used in rotary screw compressors operate in severe environments. In order to list the desired performance characteristics of compressor oils, it is important to first understand the environment the oil must withstand inside the screw compressor (see diagram I). Air enters the compressor through an air intake filter. The air compressor oil is injected into the compressor and is compressed with the air. During the air compression process heat is generated and small amounts of moisture contained in the air mix with the oil. Moisture, when mixed with oil, can cause oil water emulsions, rust and foaming. The compressed air/oil mixture then enters the air/oil separator where the hot oil and air are separated by the oil separator element. The compressed hot air then goes through a cooler, where the moisture in the air condenses and can be drained off. The air then goes to the plant to run production equipment. The hot oil goes through the oil cooler and is then injected back into the compressor once again. This process can be continuous for up to 8,000 hours or more. Oil degradation can result in acid buildup, hydrolysis and oxidation. Through the compression process the oil is expected to provide the following functions:

• Oxidation resistance from the heated air/oil compression
• Resist acid buildup due to oxidation from extended oil drain intervals, moisture and heat
• Exhibit good demulsibility to separate the water from the oil
• Prevent against internal rust formation
• Resist foaming
• Control air entrainment
• Reduce wear
• Maintain viscosity parameters
• Extend oil drain intervals
• Provide low temperature fluidity protection

Screw Compressor Diagram

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