Lubricant Base Stock Groups
Lubricant base stocks are categorized into groups by the API:
Base Oil Groups I
Group I base stocks are composed of fractionally distilled parafinic mineral oil stock which is further reined with solvent extraction processes. This improves certain properties such as oxidation resistance and removes wax, which improves the VI.
Base Oil Groups II
Group II base stock is composed of fractionally distilled parafinic mineral oil stock that has been solvent dewaxed and hydrocracked to further reine and purify the oil. Shell™ Rotella® and Pennzoil™ Purebase® are two examples of PCEO made from Group II base stocks.
Base Oil Groups III
Similar characteristics to Group II base stocks, Group III base stocks have higher viscosity indexes. Some Group III base stocks with VHVI are sometimes referred to as Group III+. Group III base stocks are produced by a wide range of processes, such as further hydrocracking of Group II base stocks, or by chemically modifying slack wax, which is the end-product of the dewaxing process, which improves the VI. Some Group III+ oils are created by highly processing oil into what have been called “Severely Hydrocracked,” “Chemically Modiied,” or “Semi-Synthetic.” Castrol™ Syntec® and Valvoline™ Durablend® are oils manufactured from Group III base stocks.
Base Oil Groups IV
This group is reserved for one type of synthetic oil made up of a class of molecules called PAOs (polyalphaoleins). Contrary to popular misconception, the molecules which make up PAO and many other synthetic oils originate in crude oil. In the case of PAOs, they are created by polymerizing olein molecules obtained from the cracking of wax molecules. In many cases, these wax molecules are a by-product of mineral oil dewaxing processes. Fully-synthetic Mobil One® and Amsoil™ are two examples of oils made from a primarily PAO base with some Group V esters added to improve additive solvent and seal swelling performance.
Base Oil Groups V
Group V is the catch-all group for any base stock not described by Groups I to IV. Examples of Group V base stocks include the old naphthenic base stocks, silicones, polyol esters, polymer esters, PAG (polyalkylene glycols), and PFPAE (perluoropolyalkylethers), as well as others. Since there is nothing in common between most of the oils lumped together as Group V, it makes no sense to discuss them as a group. Instead, a few single types of synthetic oils in Group V will be discussed individually.
Base Oil Groups VI
The Group VI class is not yet recognized in the U.S. by the API. It was introduced in Europe by the ACEA (Association des Constructeurs Europeans D’Automobiles) for the synthetic PIO (polyinternaloleins) oil base stock.