SYNTHETICS ONLY.
Historically, refiners made basestock by solvent refining and dewaxing selected crude oil fractions (Group I). With the development of hydrotreating and hydrocracking technology, refiners introduced highly refined, low aromatics, low wax basestocks with improved oxidation stability in large volume (Group II).
More recently, higher viscosity index basestocks made by high severity hydrocracking of petroleum fractions have become available (Group III). Group III stocks differ from Group II products in the structure of the lube oil molecules that impart the higher viscosity index. Group III basestocks are limited to lower viscosities, typically, 4 to 7 cSt at 100°C.
API identified polyalphaolefins (PAOs) as a special class of basestock. PAOs are made by a chemical process and have the characteristics of uniform composition, very high oxidation stability, high viscosity index and no waxy molecules. By adjusting the manufacturing process, PAOs can be made in a wide range of viscosities, commonly from 4 cSt to 100 cSt at 100°C. For many years, PAOs and esters (Group V) were the only available premium basestocks for engine lubricants operating under extreme temperature and conditions.
Most synthetic engine oils made with PAO now contain a small amount of ester to give the basestock the same solvency power as typical mineral oils. This helps to assure that the synthetic lubricant will have the same seal swell characteristics as conventional oils.
Critical base oil properties improve from Group I to Group IV. Also, less viscosity index improver (polymer) is required in multigrade engine oils with the higher VI basestocks. This leads to improved shear stability (Stay-In-Grade) and fewer deposits from polymer degradation.
Basestocks that have been severely hydrogenated to remove almost all aromatics (Group II and III) and chemically manufactured, 100 percent paraffinic PAOs (Group IV) have an added advantage in boosting the performance of dispersants in the additive package of fully formulated oils to hold soot in finely divided suspension in the oil.