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Group I base oils are typically produced using the traditional three-step process. In terms of production methods, the manufacture of Group I base oils relies mainly on physical processes that do not alter the structure of the hydrocarbons; the quality of the resulting base oil depends on the content and properties of the ideal components in the raw materials. Therefore, the performance of this type of base oil is limited. Class II base oils are produced through a combined process (a combination of solvent processing and hydrogenation), which is primarily based on chemical reactions; it is not restricted by the type of raw materials used, and it allows for the modification of the original hydrocarbon structure. Therefore, Category II base oils contain fewer impurities (with an aromatic content of less than 10%), have a high content of saturated hydrocarbons, and exhibit good thermal stability and oxidation resistance; their performance in low temperatures and in terms of soot dispersion is also superior to that of Category I base oils. Class III base oils are produced using a full hydrogenation process; compared to Class II base oils, they are hydrogenated base oils with a high viscosity index, and are also known as unconventional base oils (UCBO). Base oils of category III far exceed those of categories I and II in terms of performance, especially featuring a high viscosity index and low volatility. The performance of certain Type III oils can compare to that of polyalpha-olefins (PAO), yet their price is much lower than that of synthetic oils. Base oil of type IV refers to polyalpha-olefin (PAO) synthetic oil. Common production methods include paraffin decomposition and ethylene polymerization. Depending on their degree of polymerization, PAOs can be classified into low-polymerization, medium-polymerization, and high-polymerization types, which are used to modify different oils. Compared to mineral oils, these base oils contain no sulfur, phosphorus, or metals. Due to the absence of waxes, their pour point is extremely low, usually below –40°C, and their viscosity index is generally above 140. However, PAO has poor boundary lubricity. Furthermore, due to its low polarity, it has a poor ability to dissolve polar additives, and it also causes some shrinkage in rubber seals; however, these issues can be overcome by adding a certain amount of esters. Synthetic oils other than base oils of categories I–IV (synthetic hydrocarbons, esters, silicone oils, etc.), vegetable oils, recycled base oils, etc., are collectively referred to as base oils of category V.