Base oils are used to manufacture products including lubricating greases, motor oil and metal processing fluids. Different products require different compositions and properties in the oil. One of the most important factors is the liquid's viscosity at various temperatures.
Every lubricant used in plants today started off as just a base oil. The American Petroleum Institute (API) has categorized base oils into five categories.
- The first three groups are refined from petroleum crude oil.
- Group IV base oils are full synthetic (polyalphaolefin) oils.
- Group V is for all other base oils not included in Groups I through IV.
Before all the additives are added to the mixture, lubricating oils begin as one or more of these five groups.
| Base Oil Categories |
| Group |
Manufacturing Process |
Saturate Level |
Saturate Level |
Viscosity Index |
| Group I |
Solvent Freezing |
<90% |
>0.03% |
80-120 |
| Group II |
Hydroprocessing and Refining |
>90% |
<0.03% |
80-120 |
| Group III |
Catalytic Dewaxing |
>90% |
<0.03% |
>120 |
| Group IV |
Chemical Reactions |
100% PAOs (polyalphaolefins) |
| Group V |
As Indicated |
All others not included in Groups I, II, III or IV |