The lubricating fluid in a grease can be mineral oil (derived from petroleum), a synthetic fluid, or a vegetable based fluid. Lubricant base fluids are divided into five groups, defined by the American Petroleum Institute (API) as follows:
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- Group I – Paraffinic mineral oil, typically produced from petroleum by solvent extraction processes, with a sulfur content of >0.03% and/or saturates <90%. The viscosity index (VI) ranges from 80 to 120.
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- Group II – Paraffinic mineral oil, typically produced from petroleum by a combination of solvent extraction and catalytic processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index ranges from 80 to 120.
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- Group III – Paraffinic oil produced from petroleum by severe hydrocracking processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index is >120. Group III oils are widely (but not universally) considered to be synthetic. Gas-to-liquid (GTL) base oils are classified as Group III materials.
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- Group IV – Polyalphaolefin (PAO) fluids are the sole member of Group IV. PAOs are synthetic lubricant base stocks that are synthesized by the polymerization of linear alpha-olefins.
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- Group V – This Group includes all base stocks not covered elsewhere. It includes all synthetic fluids other than PAOs. Group V includes naphthenic mineral oils, natural esters (vegetable oils), synthetic esters, silicone oils, and all other synthetic hydrocarbons (other than PAOs).
In Europe, ATIEL defines similar categories to the API Groups, but in addition has classified poly(internal olefins), i.e., PIOs, as Group VI.