API base stock
base fluid for automotive engine oils. The API (American Petroleum Institute) currently designates five classes of base stocks. These stock types are independent of crude source or processing. Instead, they are based on physical characteristics of the base stock, which are then related to formulated engine oil performance:
| Group I | Contain <90% saturates and/or >0.03% sulfur, with viscosity index ³ 80 and <120. Commonly referred to as conventional, solvent refined base stocks. |
| Group II | Contain ³ 90% saturates and £ 0.03% sulfur, with viscosity index ³ 80 and <120. Commonly referred to as severely hydrotreated and/or hydrocracked base stocks. |
| Group III | Contain ³ 90% saturates, with £ 0.03% sulfur, with viscosity index ³ 120. Commonly referred to as VHVI or XHVI base stocks. |
| Group IV | PAO’s (polyalphaolefins) |
| Group V | All other base stocks not included in Groups I, II, III, or IV.(where < stands for less than, £ stands for less than or equal to, ³ stands for greater than or equal to, and > stands for greater than) |