Synthetic oil is a lubricant consisting of chemical compounds that are artificially made (synthesized). Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. Synthetic oil is used as a substitute for lubricant refined from petroleum when operating in extremes of temperature, because, in general, it provides superior mechanical and chemical properties to those found in traditional mineral oils. Aircraft jet engines, for example, require the use of synthetic oils, whereas aircraft piston engines do not. Synthetic lubricants are also used in metal stamping to provide environmental and other benefits when compared to conventional petroleum and animal fat based products. These products are also referred to as "non-oil" or "oil free".
Synthetic base stock lubricant oils, as described above, are man-made and tailored to have a controlled molecular structure with predictable properties. They are composed of organic and inorganic base stock oils combined with polymer packages to produce synthesised oil compounds (API Groups III, IV & V).
API Group IV Polyalphaolefins, 100% Synthetic chemical compound.
Poly-alpha-olefin (or poly-α-olefin, abbreviated as PAO), is a polymer made by polymerizing an alpha-olefin. 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.
API Group V Esters 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. 
Semi-synthetic oils (also called 'synthetic blends') are blends of mineral oil with no more than 30% synthetic oil designed to have many of the benefits of synthetic oil without matching the cost of pure synthetic oil. 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. 
The technical advantages of synthetic motor oils include:
- Better low- and high-temperature viscosity performance at service temperature extremes
- Better (higher) Viscosity Index (VI)
- Better chemical and shear stability 
- Decreased evaporative loss
- Resistance to oxidation, thermal breakdown, and oil sludge problems
- Extended drain intervals, with the environmental benefit of less used oil waste generated
- Improved fuel economy in certain engine configurations
- Better lubrication during extreme cold weather starts
- Possibly a longer engine life
- Superior protection against "ash" and other deposit formation in engine hot spots (in particular in turbochargers and superchargers) for less oil burnoff and reduced chances of damaging oil passageway clogging.
- Increased horsepower and torque due to less initial drag on engine
- Improved fuel efficiency - from 1.8% to up to 5% has been documented in fleet tests[unreliable source?]
The disadvantages of synthetic motor oils include:
- Substantially more expensive (per volume) than mineral oils.
- Potential decomposition problems in certain chemical environments (predominantly in industrial use.)
- SynLube Incorporated.  All About Synthetic Oil
- DELPHI history
- ASTM Fuels & Lubricants Handbook, Hydrocarbon Chemistry, pg 169-184, section 7
- Wills, J. George (of Mobil Oil Corporation) (1980). Lubrication Fundamentals. M. Dekker. ISBN 9780824769765.