Polymers of specific chemical monomers like esters. The first synthetic oils were developed for HCFCs (AB and PAO). With HFCs, new types of oils appeared, due to the non-miscibility of mineral oils with HFCs.
A range of PAG (polyalkylene glycol) oils specifically formulated to operate in CO2 systems (R-744). Zerol RFL-EP oils have excellent thermal stability, and are ideal for subcritical and transcritical operation.