Evon Royal, Contributor 
2006 Hyundai Azera.
SYNTHETIC OILS are basically made either from liquid petroleum or solid petroleum; both being crude material found in the ground. When solid petroleum particles and hydrogen gas are combined at very high temperature and pressure in the presence of a catalyst they synthesise to form liquid hydrocarbon.
Different types of synthetic hydrocarbons can be made by this method but the most common type is Poly Alpha Olefin (PAO) oil. PAO oil has been manufactured for several decades and is the base oil in many popular brands of synthetic motor oil.
In a somewhat similar method called hydro-cracking, crude liquid petroleum can be processed into a lubricating oil by blasting hydrogen into it at very high temperature and pressure in the presence of a catalyst. However, to produce a synthetic oil waxy perafinic oil is selected and in two, combined processes called hydrocracking-hydroisomerisation processed into very High Viscosity Index (VHVI) oil. VHVI oil is more recently manufactured but is rapidly becoming as popular as PAO oil.
PAO and VMVI oils have very high purity. These oils are totally free of nitrogen, oxygen, sulphur and other impurities which break up the molecular structure of hydrocarbons at high temperatures and cause the carbon to break free and build up in the engine.
VISCOSITY INDEXES
Probably the most outstanding property of PAO and VHVI oils are their very high viscosity indexes. The viscosity of an oil decreases when its temperature increases. The viscosity of a very high viscosity index oil decreases with increasing temperatures by an amount which is much less than the amount by which the viscosity of a law viscosity index oil decreases with the same increasing temperatures. Though the name VHVI may suggest otherwise the viscosity index of PAO oil is higher than that of VHVI oil.
The film strength of VHVI oil can be increased by selecting liquid petroleum which naturally has very high film strength. This option does not exist in the manufacturing of PAO oil. The film strength of PAI oil is usually strengthened with synthetic ester. The use of ester in motor oils is controversial. Maybe it is for this reason that some oil companies produce only VMVI oil as their synthetic motor oil and some luxury sports car companies recommend only VHVI synthetic motor oil for their engines.
It is not the intention of this article to knock PAO oil. PAO and VHVI oils have served motorist excellently in the past and by all indications will continue that trend.
Evon Royal is a mechanical engineer
