Originally Posted by nthach
The rationale for the automakers using PAG oil from what I've read is that it doesn't degrade like an POE does upon contact with water or moisture into acids. Funny enough, POE oil is being used in hybrids and BEV vehicles - POE oil has better dielectric qualities than PAG.
PAGs made the first inroads into automotive air conditioning units and the systems were designed around that chemistry. When POEs later became the chemistry of choice for HFC refrigeration systems it was deemed too expensive to re-engineer the automotive AC systems, and the PAGs were working fine, so they stayed with PAGs for new builds. For existing cars and trucks with mineral oil in the AC systems, PAGs were considered not suitable for fluid changes and top-offs due to their incompatibility with the mineral oils left in the system unless the entire system was drained and purged. POEs, however, are compatible with both PAGs and mineral oils and therefore became the dominant chemistry in changeovers and top-off kits for all cars since they are friendly with all residual oil chemistries. There were a few automakers that did switch to POE systems but I don't know the current status.
In the early days of the refrigerant and lubricant changes, some problems occurred with clogged capillary tubes in some refrigeration systems. The deposits were found to be metal carboxylates and were assumed to come from hydrolysis of POEs. This led to the development of hydrolytically stable POEs, but it was later found that the deposits actually came from residual metal metalworking fluids and not the POEs. While the hydrolytically stable POEs are still used by some compressor builders, the majority of POE refrigeration oils today are not of that chemical structure and work fine. The POEs are dried to a very low moisture content of < 50 ppm and hydrolysis is not an issue in this application.
