The moving parts of a compressor must be lubricated during operation. The oil is absorbed into the refrigerant that carries it through the compressor as a mist. An oil over/under charge can damage the compressor. Compatibility of refrigerant oil is determined by its ability to remain oil when mixed with the refrigerant and not become separated by a chemical reaction. R12 systems use a non-foaming sulfur-free grade of mineral oil (YN-9) formulated for that refrigerant and cannot be used in R134a systems. R134a systems use either synthetic oil (PAG) YN-12 or Double End Capped PAG oil.