"Why do some people use automatic transmission fluid to replace compressor oil in certain portable compressor units? What are the drawbacks of such a replacement?"
Automatic transmission fluid (ATF) is a common lubricant found in many compressors. Although using an oil that is designed for automotive applications in industrial equipment could cause some concern, this particular combination of equipment needs and lubricant properties is well-aligned. Many of the same performance characteristics that make ATF ideal for transmissions also make it well-suited for the rigors of a compressor system.
Within a transmission, the lubricant must perform a variety of tasks, such as cooling parts, keeping them free of deposits, serving as a hydraulic medium, reducing friction and wear, and resisting a breakdown under the stress of operation. Therefore, many ATFs are formulated with robust base oils and enhanced additive packages that contain anti-wear agents, antioxidants, detergents and demulsifiers. This allows ATFs to have long service intervals and be changed only after thousands of miles of driving.
Many of these same stresses exist within compressors. Again, the fluid must resist breaking down in the presence of heat, as oils that break down can lead to deposits being formed in the high-heat areas of the compressor. The detergency found in ATF helps to reduce these deposits, resulting in cleaner internal parts and a more efficient compressor. In addition, as gas is compressed, it is driven into the oil, which could cause oxidation and premature oil failure. ATFs have base oils and antioxidants that resist this type of degradation, enabling longer drain intervals and more consistent lubricant properties over the life of the oil.
While it may sound as if an ATF could work well in every application, there are some drawbacks or risks that must be considered. As with most lubricants, there is a chance of incompatibility between the in-service lubricant and the new oil being added to the system. For ATFs, not only could the base oil cause incompatibility issues but also the additives. ATFs typically have synthetic base oils, so always perform a thorough flush before using them in compressor applications.
Some compressor manufacturers have strict guidelines as to which oil must be used during the warranty period. If your compressor is still under warranty, avoid switching lubricants. Even though an ATF may work perfectly, voiding a warranty could be costlier than any savings achieved by using a cheaper lubricant.