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:
- Group I – Paraffinic mineral oil, typically produced by solvent extraction processes, with a sulfur content of >0.03% and/or saturates <90%. The viscosity index is in the range from 80 to 120.
- Group II – Paraffinic mineral oil, typically produced by a combination of solvent extraction and catalytic processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index is in the range from 80 to 120.
- Group III – Paraffinic oil produced by severe hydrocracking processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index is >120. Group III oils are considered to be synthetic. Gas-to-liquid base oils are classified as Group III materials.
- Group IV – Polyalphaolefins. This group is dedicated only to polyalphaolefin (PAO) fluids. PAO is a synthetic lubricant base stock.
- Group V – This group covers all base stocks not covered elsewhere. It includes all synthetic fluids other than PAO, as well as naphthenic mineral oils.