API Base Stock Groups:
Lubricant base stocks are categorized into five groups by the API:
Group I base stocks are composed of fractionally distilled petroleum which is further refined with solvent extraction processes to improve certain properties such as oxidation resistance and to remove wax. These oils are no longer very common and should never be used in an automotive engine.
Group II base stocks are composed of fractionally distilled petroleum that has been hydrocracked to further refine and purify it. These oils comprise many of the single grade motor oil you see on the shelf.
Group III base stocks have similar characteristics to Group II base stocks, except that Group III base stocks have higher viscosity indexes. Group III oils are produced by further hydrocracking of Group II base stocks, or of hydroisomerized slack wax. Manufacturers label Group III oils as standard motor oil or as synthetic in the US for marketing purposes. It depends on the level of refinement and additive pack. The ACEA in Europe does not allow a Group III oil to be labeled as “synthetic”.
Group IV base stocks are polyalphaolefins (PAO). These were the 1st synthetic oils produced.
Group V is a catch all group for any other synthetic base stocks. Examples of Group V base stocks include polyol esters (POE), polyalkylene glycols (PAG oils), and perfluoropolyalkylethers (PFPAEs). In addition, base stocks using Fischer-Tropsch-synthesis (gas-to-liquid) fall in this group.
POE oils are the most common Group V oils (Red Line for example), in fact, POE oils are used for lubrication in jet turbines and are some of the best oils available. All the Group V oils represent the latest in petroleum technology. But, just because oil is classified as Group V doesn’t mean that it’s better….Group V oils are simply synthetics that are not PAO based.