The Synthetic Myth

What do we mean by the word 'synthetic'? Once, it meant the 'brick by brick' chemical building of a designer oil, but the waters have been muddied by a court case that took place in the USA a few years ago, where the right to call heavily-modified mineral oil 'synthetic', was won. This was the answer to the ad-man's dream; the chance to use that sexy word 'synthetic' on the can… without spending much extra on the contents! Most lower-cost 'synthetic ' or 'semi-synthetic' oils use these 'hydro-cracked' mineral oils. They do have some advantages, particularly in commercial diesel lubricants, but their value in performance engines is marginal.

True synthetics are expensive (about 6 times more than top quality mineral types). Looked at non-basically, there are three broad categories, each containing dozens of types and viscosity grades:-

1) PIB's (Polyisobutanes)
These are occasionally used as thickeners in motor oils and gear oils, but their main application is to suppress smoke in 2-strokes.

The two important ones are:-

2) Esters
All jet engines are lubricated with synthetic esters, and have been for 50 years, but these expensive fluids only started to appear in petrol engine oils about 20 years ago. Thanks to their aviation origins, the types suitable for lubricants (esters also appear in perfumes; they are different!) work well from -50º C to 200º C, and they have a useful extra trick. Due to their structure, ester molecules are 'polar'; they stick to metal surfaces using electrostatic forces. This means that a protective layer is there at all times, even during that crucial start-up period. This helps to protect cams, gears, piston rings and valve train components, where lubrication is 'boundary' rather than 'hydrodynamic', i.e. a very thin non pressure-fed film has to hold the surfaces apart. Even crank bearings benefit at starts, stops, or when extreme shock loads upset the hydrodynamic film. (Are you listening, all you rally drivers and off-road fanatics?)

3) Synthetic Hydrocarbons or PAO's (Poly Alpha Olefins)
These are, in effect, very precisely made equivalents to the most desirable mineral oil molecules. As with esters, they work very well at low temperatures, and equally well when the heat is on, if protected by anti-oxidants. The difference is, they are inert, and not polar. In fact, on their own they are hopeless 'boundary' lubricants, with less load carrying ability than mineral oil. They depend entirely on the correct chemical enhancements.

In fact PAOs work best in combination with esters. The esters assist load carrying, reduce friction, and cut down seal drag and wear, whilst the PAOs act as solvents for the multigrade polymers and a large assortment of special compounds that act as dispersants, detergents, anti-wear and anti-oxidant agents, and foam suppressants. Both are very good at resisting high-temperature evaporation, and the esters in particular will never carbonise in turbo bearings even when provoked by anti-lag systems.

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