All oils are comprised of basestocks and additives. Basestocks make up the majority of the finished product and represent between 75-95%.
Not all basestocks are derived from petroleum, in fact the better quality ones are synthetics made in laboratories by chemists specifically designed for the application for which they are intended.
Basestocks are classified in 5 Groups as follows:
Group I
These are derived from petroleum and are the least refined. These are used in a small amount of automotive oils where the applications are not demanding.
Group II
These are derived from petroleum and are mainly used in mineral automotive oils. Their performance is acceptable with regards to wear, thermal stability and oxidation stability but not so good at lower temperatures.
Group III
These are derived from petroleum but are the most refined of the mineral oil basestocks. They are not chemically engineered like synthetics but offer the highest level of performance of all the petroleum basestocks. They are also known as “hydrocrackedâ€