General Description of Base Oil
In 1990 API established a base oil classification system to help marketers minimize re-testing costs when blending licensed engine oils with base oils from different manufacturing sources. The system uses physical and chemical parameters to divide all base stocks into 5 groups as listed below:
API – Classification of Base Oils
| Group | Saturate wt % | Sulphur wt % | Viscosity Index | |
| I | < 90 and/or | > 0.03 | > 80 to <120 | Paraffinic |
| II | >90 and | < 0.03 | > 80 to <120 | Paraffinic |
| III | >90 | > 0.03 | >120 | Paraffinic |
| IV | All poly Alpha Olefins (PAOs) | Lubricants property improves | ||
| V | All Base Stocks not incl. in Groups I- IV | Naphthenic – Synthetic esters | ||