In terms of definition, the solvent-extracted mineral base stocks are classified as Group I, for basically untreated mineral base oil; a Group II lubricant is subjected to a severe hydrotreating which gets rid of most of the unsaturated molecules and makes the oil much more stable. A further hydrotreating creates a more refined Group III stock with a higher viscosity index (VI). The pure synthetic PAO oil is Group IV, and all the other lubricant stocks not covered by these classifications are in Group V.
by Steve Sturgess and Rolf Lockwood