The industry standard is based on the API Base Oil Classification:
Group I – Conventional Paraffinic Mineral base oils
Group II – Low sulphur Paraffinic and some hydro-cracked (VI below 120) mineral base oils
Group III – Hydro-cracked (VI above 120) synthetic base oils
Group IV – Poly-Alpha-Olefins (PAO) synthetic base oils
Group V – Esters etc.