There are five specific categories of base oils. These categories define the type of base stock the oil is formulated from. Group I, II, II+ and III type mineral base oil stocks are widely used in combination with additive packages, performance packages, ester and/or Group IV Polyalphaolefins in order to formulate semi synthetic based lubricants. Group III base oils are sometimes considered as synthetic but they are still classified as highest top level mineral base stocks. A Synthetic or Synthesized material is one that is produced by combining or building individual units into a unified entry. Synthetic base stocks as described above are man-made and tailored to have a controlled molecular structure with predictable properties, unlike mineral base oils which are complex mixtures of naturally occurring hydrocarbons.