Posted by Wikipedia on October 04, 2016
Base oil groups
Manufactured by solvent extraction, solvent or catalytic dewaxing, and hydro-finishing processes. Common Group I base oil are 150SN (solvent neutral), 500SN, and 150BS (brightstock)
Group II – Saturates > 90% and sulfur < 0.03%, and SAE viscosity index of 80 to 120
Manufactured by hydrocracking and solvent or catalytic dewaxing processes. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.
Group III – Saturates > 90%, sulfur < 0.03%, and SAE viscosity index over 120
Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing process.
Full synthetic oils are derived from crude oil or byproducts through a chemical process rather than the distillation process used for conventional oil. The resulting synthetic oil has a more consistent molecular size and therefore tends to have somewhat more uniform properties. The base oils are categorized by American Petroleum Institute into API Groups III, IV & V.
Group IV: PAO
Poly-alpha-olefin (or poly-α-olefin, abbreviated as PAO) is a polymer made by polymerizing an alpha-olefin. They are designated at API Group IV and are a 100% synthetic chemical compound. It is a specific type of olefin (organic) that is used as a base stock in the production of some synthetic lubricants. An alpha-olefin (or α-olefin) is an alkene where the carbon-carbon double bond starts at the α-carbon atom, i.e. the double bond is between the #1 and #2 carbons in the molecule.
Group V: Other Synthetics
Esters are the most famous synthetics in Group V, which are 100% Synthetic chemical compounds consisting of a carbonyl adjacent to an ether linkage. They are derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group, most commonly from carboxylic acids and alcohols. That is to say, esters are formed by condensing an acid with an alcohol.
Many chemically different "esters" due to their usually excellent lubricity are used for various reasons as either "additives" or "base stocks" for lubricants. 
The lubricant industry commonly extends this group terminology to include:
Group I+ with a Viscosity Index of 103–108
Group II+ with a Viscosity Index of 113–119
Group III+ with a Viscosity Index of at least 140
Can also be classified into three categories depending on the prevailing compositions:
Lubricants for internal combustion engines contain additives to reduce oxidation and improve lubrication. The main constituent of such lubricant product is called the base oil, base stock. While it is advantageous to have a high-grade base oil in a lubricant, proper selection of the lubricant additives is equally as important. Thus some poorly selected formulation of PAO lubricant may not last as long as more expensive formulation of Group III+ lubricant.
Semi-synthetic oils (also called "synthetic blends") are a mixture of mineral oil and synthetic oil, with no more than 30% synthetic oil, which are engineered to have many of the benefits of full synthetic oil without the cost. Motul introduced the first semi-synthetic motor oil in 1966.
Lubricants that have synthetic base stocks even lower than 30% but with high-performance additives consisting of esters can also be considered synthetic lubricants. In general, the ratio of the synthetic base stock is used to define commodity codes among the customs declarations of tax purposes.
Other base stocks help semi-synthetic lubricants
API Group II- and API Group III-type base stocks help to formulate more economic-type semi-synthetic lubricants. API Group I-, II-, II+-, and III-type mineral-base oil stocks are widely used in combination with additive packages, performance packages, and ester and/or API Group IV poly-alpha-olefins in order to formulate semi-synthetic-based lubricants. API Group III base oils are sometimes considered Fully 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 and paraffins.
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