In the 1930s, Dr. Hermann Zorn of Germany was searching for a lubricant with the properties of natural oils derived from crude oil but without the undesirable properties (high pour points, tendency to gum or gel in combustion engines, low oxidation resistance at higher temperatures, etc.). Germany was also in need of a product that was not derived from crude oil, as the nation’s access to crude oil was becoming increasingly scarce. By the mid-1940s, the fruit of Dr. Zorn’s labor included more than 3,500 different blends of esters, including diesters and polyolesters.
The first real-world trial for these lubricants came during World War II when both Germany and U.S. forces began using synthetic base oil in aircraft engines. They noticed the synthetics made engine starts much easier in colder climates (due to the high viscosity index) and significantly decreased soot deposits that would build up in oil radiators when using conventional (crude oil-derived) lubricants.
There are two American Petroleum Institute (API) base oil categories that include synthetics. The first is API Group IV. The only synthetic base oil included in this group is polyalphaolefin or PAO. PAOs are made by polymerizing an alpha-olefin molecule like ethylene. In an alpha-olefin molecule, there is a carbon-carbon double bond with hydrogen branching off.
The second category is API Group V. These are non- PAO synthetic bases. Examples include diesters, polyolesters, alkylated benzenes, phosphate esters, etc. Basically, if it is a synthetic and it is not a PAO, it is a Group V.
Some confusion has arisen recently regarding the use of the word “synthetic.” Several petrochemical companies have developed processes involving catalytic conversion of crude oil base stock under high pressures and temperatures in the presence of hydrogen to form very high-quality mineral lubricants. These oils, which are known as API Group III, are so highly refined that their properties almost match that of the Group IV synthetics. They are so close in fact that the U.S. court system sided with a manufacturer of these Group III “synthetics” when a lawsuit was brought up for false advertising. Even though these Group III base oils are derived from crude oil, they can now legally, from a marketing standpoint, call them synthetic.
When designing a lubrication program, I use a very simple set of rules to know when to choose a synthetic for an application. They are as follows: