Base oils are very different from each other in molecular components and therefore in physical and chemical properties (eg viscosity-temperature properties). For this reason, in 1993, the American Petroleum Association (API) designed an oil classification system and divided all base oils into 5 groups based on the weight percentage of saturated compounds, the weight percentage of sulfur, and the viscosity index, as shown in the table below. It should be noted that the base oil is classified only on the basis of physical and chemical properties and not on the basis of production method. This means that the production method alone does not reflect the level of quality of the base oil. To improve the quality of the base oil, the percentage of unsaturated compounds and sulfur in it should be reduced and the viscosity index of the oil should be increased. To achieve this goal, various refining processes are used By moving from group I to III, the percentage of paraffins increases. Group 1 base oils are different from Group 2 and Group 3 has significantly less impurities (less than 30 ppm sulfur and less than 10% unsaturated compounds) and base group oils (Group 3) are somewhat pure. Which are almost colorless. Group 3 oils are also called conventional oils or base oils with very high viscosity index (VHVI). Group 4 includes all polyalphaolefins (PAOs) that are used in pure form as well as mixed with mineral base oils for lubrication conditions. Group 5 includes all other base oils, including all naphthenic base oils, medium viscosity paraffin oils, and synthetic liquids such as esters, silicones, polyglycols, and vegetable oils.