PAG oil, or Polyalkylene Glycol, is a fully synthetic hygroscopic oil specifically designed for automotive air conditioner compressors. It is used in R-134a air conditioning systems to lubricate the compressor. When looking at PAG oil you will notice various numbers such as PAG46 or PAG100. These numbers refer to the viscosity of the oil, similar to 10W30 oil. In order to determine the correct PAG viscosity for your vehicle you will need to look up the specifications of your make and model of your vehicle either online or in the instruction manual.