The two common oils in heavy-duty mobile R134a systems are PAG (polyalkelene glycol) and POE (polyolester). If you sell oil or service A/C systems, it’s important to understand the difference.
PAG: Not All Are Equal
Polyalkelene glycol (PAG) is a synthetic oil used by most compressor manufacturers for their original fills. PAGs come in a range of viscosities and additive packages and are available from several suppliers. Additives are typically added to the base lubricant to enhance properties such as lubricity or thermal stability.
Suppliers will “end-cap” PAG, altering the structure of its polymers. A “double” end-capped PAG is one with no alcohol groups on the ends of its polymer chain. This greatly increases its stability and moisture tolerance. Even at high temperatures, the end-capped PAG will not react with moisture to form harmful acids.
POE: Beware of Hydrolysis
Polyolester is a popular synthetic aftermarket lubricant because it is compatible with R134a, R12, and R410a. Hydrolysis is the main issue with POE oils: in the presence of water, it will decompose into acid and alcohol.
The degree of hydrolysis is driven by the amount of water present; the speed at which hydrolysis occurs is dependent on temperature and the acid content (acids can act as a catalyst). Either way, the result is bad news for a moisture-contaminated A/C system.
PAGs and POEs both mix and circulate with R134a. Whether or not they will provide adequate lubrication is the issue.
Always check with the compressor OEM to verify the type and viscosity of oil you need. Using too much or too little oil, or the wrong type, can lead to increased wear, premature component failure, and the loss of your warranty.
If the OEM lubricant is not available, a double end-capped PAG of the same ISO viscosity as the original is the best choice.