An apt definition of a Synthetic Base Oil would be any Base Oil used for lubrication, that does not include, or is not derived from crude oil. They are produced chemically, with the aim of performing in a much better manner than Group I, Group II and Group III mineral-based Base Oils. The molecular structures of Synthetic Base Oils are tailor-made to be smaller, uniform and more stable (vs. mineral-based oils). There are also lesser contaminants as compared to conventional base oils.
While there is a very broad range of Synthetic Base Oils, the most commonly used ones are Poly-Alpha Olefins (PAO), also known as Synthetic Hydrocarbons (SHC), Polyglycols, and Esters, finding good use in Engine Oils, Gear Oils, Air Compressor Oils, Fire-Resistant Hydraulic Fluids, and other applications wherein temperatures and pressures are at an extreme.
In general, as opposed to mineral-oils, Synthetic Base Oils possess the following advantages,
1.) High Viscosity Index allowing operations at higher temperatures, and excellent flow properties at start-up
2.) The High VI and uniform molecular structure also ensure greater friction reduction properties, especially at extreme load and high-temperature applications.
3.) Lower volatility equating to lesser top-ups or consumption
4.) Higher oxidation stability ensuring a significant improvement in oil-life
Costs are obviously higher, but one must weigh the cost vs the benefits (ex. longer component life) before making a decision in favor of mineral based-oils or Synthetics.
As a final note, care must be given to products labeled as “semi-synthetic” or “synthetic-blends”, as these products are ‘partial synthetics’ only, wherein the finished product will contain ‘some’ PAO or Esters, with the balance as mineral oils. The performance will vary.