Larry Crider
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There are literally hundreds of types of synthetic lubricants and hundreds more variations of these! All custom tailored for specific uses from automotive to industrial to aviation

Here I will tell you about the most common synthetic lubricants used for automotive, fleet and some industrial uses and how they relate to the petroleum oils most people are familiar with. 

The components of a synthetic lubricants base stock are man made designer molecules "synthesized" by chemically reacting two or more simpler compounds two produce a finished base stock. 

This allows a lubrication engineer the ability to tailor the base stock to a specific application for maximum lubricant performance. 

To understand how synthetic lubricants relate to today's marketplace we must we run through the list of available oils. In the "oil bidness" different oils are categorized in-groups by the type of base stock they are made from. The base stock is just that. The bulk of the product you see in the bottle minus the additives that is mixed in to make the oil perform certain functions. 

Group one oil: 

These are the old high paraffin base oils. They are not used for modern engine oils anymore but can still be found in those little bottles of all-purpose household oil you buy at the hardware store for stuff like oiling door hinges. 

Group two oils: 

This is the standard petroleum base stock that all modern conventional petroleum oils are made from. Quality varies widely depending on where it was "dug up from". Even in "finished" form it can contain various amounts of paraffin (wax), impurities left over from refining and from the ground it came from! Chemically it is a hodge podge of different sized hydrocarbon molecules, not all of which "get along with each other chemically" so to speak. The result of this is a product that produces sludge, varnish and mechanical wear as it ages and breaks down in service. These oils have steadily been improved over the years as API services requirements have gotten stricter. However as modern engines pump more horsepower from smaller engines with less total oil sump capacity and the level of horsepower/torque transmitted though today's light weight fuel efficient drive trains continues to climb, lubricant manufacturers find that conventional petroleum oils really just cannot be improved any farther. Hence the move we see by automakers to synthetic fluids; both in engines and transmission/differentials. Combine that with the need to improve fuel economy and synthetic lubricants well know ability to do just that and you can see why "factory filled" with synthetic is becoming more and more common. 

Group three oils: 

Group three oils are synthetic oils that are derived from petroleum base stocks that have been hydroisomerized, and chemically altered to produce a lower cost synthetic base stock.

Although it derives from petroleum base oil and is not made from a synthesized, engineered molecule and as such is sometimes not considered true synthetic oil, it does offer a lot of the benefits one would expect from a engineered synthetic base oil. 

Group three based synthetic oils out perform there petroleum brethren on any number of industry tests  and in fact are usually sold and marketed as a 100% synthetic product. 

In fact the vast majority of synthetic oils on the market are actually made from group three oils because of a lawsuit a few years ago between Mobil and Castrol that totally changed the synthetic oil industry. 

Because of this lawsuit the buying public has largely come to  consider  these oils to be true synthetic. 

Here's what happened. Exxon- Mobil Inc. makers of Mobil One sued Castrol, makers of Castrol Syntec, accusing them of marketing a hydro cracked petroleum oil as a synthetic-which they were! 

Mobil Inc. felt that Castrol Inc. had pursued an unfair market advantage because group three based oils are much less expensive to manufacture than true synthetic oils yet Castrol was marketing Syntec as a 100% synthetic product: Castrol could make it for less, sell it for less and un-fairly under cut all it's competitors in the synthetic oil market with a oil that was not truly a synthetic product. 
In the end though, Castrol convinced the court that group a three-based oil has been sufficiently refined and chemically altered that it should be able to be marketed along with true synthetic oils. Basically the court expanded the definition of synthetic to include group three based oils. 

Because the synthetic oil market is the fastest growing part of the lubricants industry, manufactures are eager to jump up and grab the profits that having that sexy customer grabbing word synthetic on the bottle bring. 

While Group three oils do not have the performance of true custom engineered synthetic base oils such as PAO and Ester based synthetic oils, especially in temperature extremes where the men get separated from the boys so to speak…they do offer very good performance at a price somewhat less than the more expensive group 4 and group 5 based synthetic oils 

Group four oils: 

Polyalphaolefln and related olefin oligomers and olefin polymers. (Synthetic hydrocarbon) PAO's as they are commonly called are a true man made engineered base oil produced by catalytic reaction with various alpha olefin compounds. 

PAO's are widely recognized as providing outstanding performance for many lubricant applications because of its very high viscosity index, a wide operational temperature range and because it is thermal and shear stable. 

PAO's also have low corrosivity and are compatible with mineral oils and the range of materials that engines and other machinery is manufactured from. 

The molecular structure of PAO's are easily customized for use in all kinds of applications from automotive to industrial and are widely used in motor oils, gear lubes, high temperature/extreme pressure greases, Compressor oils and hydraulic fluids. 

Group five oils: 

This group comprises all synthetic oils other than PAO's. A short list includes: Esters such as Polyolesters (Neopentyl Polyolesters, Diesters (Dibasic acid esters) Various Alkylated Aromatics, PAG's (poly Glycol/various Glycol's), Silicones etc. 

This group of synthetics is primarily used for various industrial and aviation applications. Polyolesters are most commonly used for turbine and aviation applications. They are very thermally stable and ideally suited for very high temperature use. Hence there use in such things as high temp greases, jet engines and gas turbines. 

They have a very low coefficient of friction and are sometimes added in small amounts to mineral oils and other synthetic oils to lower the coefficient of friction of the finished product. 

Diesters are most commonly used aviation and industrial compressor applications however because of the tremendous anti-scuffing protection they offer. They are often added in small amounts to PAO based two-cycle oils as this feature is very beneficial to a two-cycle motor oil. 

Diesters are shear stable, have good lubricity, detergency and are polar meaning they have an electrical charge that causes them to cling to metal surfaces-a desirable trait for most lubricants. Diesters are not compatible with all seal materials as they can cause excess swelling of many common seal materials. Because of this they are often added in small amounts to automotive PAO based oils to provide positive seal function. 

Silicones offer wide temperature performance and are sometimes used in compressor applications although this use has fallen out of favor in recent years. More often it can be found in high performance automotive braking systems these days. 

By Larry Crider , Lubrication Specialist