All lubricants consist of Base Oil. Normally, lubricants consist of 90% base oil and 10% additives. The American Petroleum Institute’s (API) has categorized base oils into five groups which are specified by the saturate level, Sulfur level, and viscosity index.
Saturates are a type of molecule commonly found in base oil. They are naturally present in base oil but during the refining process higher levels of saturates are obtained. If the level of saturates is higher, the molecular bond of the oil is stronger. This will increase the resistance to breakdown and oxidation or the loss of viscosity.
Sulfur is a natural inorganic element occurring in crude oil. Because it reacts with oxygen it can be harmful to the performance of oil. It can also be damaging to exhaust after treatment devices. Besides these negative aspects of Sulfur there are also some positive aspects. Sulfur can be an effective antioxidant which improves the oxidative stability. When the content of sulphur is lower, the purity is better which decreases the probability of corrosion and oxidation.
The Viscosity Index refers to the changes in viscosity compared to the temperature of the oil. The viscosity is measured at 40 °C and 100 °C. When the viscosity index is high, the changes are smaller with differences in temperature. All oils increase in viscosity when the temperature decreases and decrease in viscosity when temperatures increase.