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The American Petroleum Institute (API) groups oils into several categories, roughly defined as: Group I is mineral oils; Group II is hydro-treated oils; Group III is hydrocracked oils. Synthetics generally make up Groups IV and V. Group IV is polyalphaolefins (PAO). Group V contains other synthetics such as polyalkylene glycols (PAG), diesters, polyolesters (POE), silicones, perfluoropolyethers (PFPE), alkylated benzenes and phosphate esters.
Though many different base stocks make up the synthetic hydrocarbon family, the majority of modern process plants make primary use of fluids with PAO and diester base stocks. PAOs are touted as having higher oxidative stability and providing superior performance to mineral oils in demanding applications. PAGs are said to have superior deposit control over mineral and PAOs. Synthetics also provide the benefit of temperature stability.