R-134a A/C refrigerant guide of Subaru cars:

Refrigerant Oils

Let’s take a look at the lubrication and type of oil that is recommended for use with the new refrigerant. Service technicians should understand the differences between these lubricants.

R-134a systems use a synthetic oil known as polyalkaline glycol, or PAG. This oil mixes well with R-134a and circulates throughout the A/C system. Due to possible differences in chemical makeup PAG oil from one manufacturer should never be mixed with other types of PAG oil.

Unfortunately, and most important of all, is that mineral-based oils, the type used with R-12 systems, will not mix with R-134a refrigerant.Therefore, mineral based oil will not effectively lubricate an R-134a system.

An important point to remember is that PAG oil will absorb moisture at a much greater rate than mineral-based oil. In fact, PAG oil’s capacity for moisture absorption is 1000 times greater than that of mineral-based oils. Proper “capping” of the compressor and lines while servicing must be done to protect the air conditioning system.

PAG oil should never be left in an open container in the shop area. The moisture it absorbs could form compounds that will damage the air conditioning system.

Due to the increased risk of moisture absorption, and to be compatible with R-134a refrigerants, the new A/C systems will use an R-134a specific receiver drier. This drier uses a new type of desiccant.

The amount of desiccant used in an R-134a A/C system has also been increased. Generally speaking, the receiver-drier looks the same, and it serves the same purpose, but again critical differences do exist and proper identification is important.