Before looking at the differences between the oils on the shelf, it is worth remembering that oil quality is an important factor in your engine's health.
As well as lubricating the pistons, it also has to oil the turbo bearing, which spins at tens or even hundreds of thousands of revolutions per minute. You will have gathered that at such speeds it is vital for the oil to be beyond reproach. It would even be interesting to know how many blown turbos are caused by poor lubrication (low-quality oil, or oil too old and past its properties, and so on). So if your engine is turbocharged, oil quality matters all the more.
Finally, bear in mind that an engine wears most when cold: an oil that performs well at low temperatures will do much to reduce that.
Synthetic, mineral and semi-synthetic
What is mineral oil?
Mineral oil comes from refining crude oil, which in its natural state is a complex mixture of hydrocarbons whose molecules have a wide variety of lubricating characteristics: some good, others less so. Refining starts by removing most of the natural contaminants and unwanted hydrocarbons present in the crude. Some “undesirables” remain, however, such as wax — a poor lubricant and a substance that reacts badly to heat.
The hydrocarbon chains that make up mineral oil vary in length (which is what determines its viscosity), and their molecules are held together by chemical bonds that can oxidise at high temperatures. Additives therefore have to be added to remove or reduce the harmful effects of impurities on the lubricant. The finished product in fact contains between 10 and 25% additives.
What is synthetic oil?
The hydrocarbons in synthetic oil, by contrast, are not of natural origin. This oil is manufactured (or synthesised) from any source of carbon and hydrogen, such as vegetable oil or ethylene. In preparing it, chemists set out to obtain a pure product, free of contaminants and wax, rigorously uniform, and whose molecular chain length matches the target viscosity exactly. The result? An absolutely homogeneous product.
The benefits of synthetic oil
Synthetic oil's first advantage is that it offers a far wider operating temperature range than mineral oil. Mineral oil can generally become solid from -30°C, whereas synthetic oil still flows at -50°C. And if an oil solidifies at -30°C, it starts thickening well before that limit. Synthetic oil is therefore well suited to our winters: being more fluid, it lubricates the engine almost instantly.
Heat is one of oil's worst enemies. Above 245°C, mineral oil quickly volatilises, forming varnish, sludge and tar while losing its lubricating qualities, whereas synthetic oil keeps its properties up to 370°C. Moreover, a six-and-a-half-hour test at 205°C showed that mineral oil loses more than 20% of its weight to evaporation in those conditions, against 4% for synthetic. That matters when you know temperatures can reach 315°C in the upper parts of the engine and 205°C (400°F) at the piston rings.
Unlike mineral oil, which tends to bead on metal at very high temperatures, synthetic oil adheres to it better and, in doing so, dissipates heat more effectively. This superior adhesion — and therefore better lubrication — is particularly valuable for drivers who only use their car occasionally.
Synthetic oil is an oil with superior properties, giving consumers a quality option for protecting their engine components. It is also worth noting that some manufacturers require synthetic oil in their engines.
