Grades of oil.
Motor oils are derived from base stocks. That is, a generic oil base is modified with additives to produce a lubricant with the desired properties. A base stock oil with no additives would not perform very well at all. Base stocks are classified by the American Petroleum Institute (API) and fall into one of five categories.
Group I and II - these are mineral oils derived from crude oil
Group III - this is a highly refined mineral oil made through a process called hydrocracking. In North America this group is considered a synthetic oil, for marketing purposes.
Group IV - these are true synthetic oils, known as Polyalphaolefin (PAO).
Group V - these are synthetic stocks other than PAO's and include esters and other compounds.