The API defines the difference between Group II and III base oils only in terms of V.I., viscosity index. Base oils with conventional V.I. (80-119) are Group II and base oils with an “unconventional” V.I. (120+) are Group III. Group III base oils are also called unconventional base oils (UCBO’s) or very high V.I. (VHVI) base oils. Group II+ base oils have the same maximum V.I. as Group II (80-119), but have a higher minimum V.I. (110-119).
From a process standpoint, Group III oils are made by the same process as Group II oils, but the V.I. is increased by increasing the temperature of the hydrocracker. The product V.I. can also be increased by increasing the V.I. of the feedstock. Which is done by selecting the appropriate crude.
