Mineral, semi-synthetic, fully synthetic

Every motor oil consists of two parts

A finished motor oil is base oil plus additive package. The base oil makes up the largest share and determines flow behaviour, temperature stability and evaporation loss. The additive package brings everything the base oil cannot do on its own: detergents and dispersants keep soot and deposits in suspension, anti-wear additives form a protective layer at highly loaded points, antioxidants slow down ageing, and viscosity index improvers make sure a multigrade oil works across the whole temperature range. Two oils with the same base oil can therefore behave completely differently.

The base oil groups

• Groups I and II are mineral base oils, obtained directly from refining crude oil and purified to varying degrees.
• Group III is produced by hydrocracking. The result is a very pure, thermally stable oil that may be labelled fully synthetic in Europe even though it comes from crude oil.
• Group IV comprises polyalphaolefins, genuine synthetic molecules built up in the laboratory with particularly good cold behaviour.
• Group V covers everything else, above all esters, which are blended in for adhesion and ageing stability.

This also explains why two fully synthetic oils can differ so much in price: an oil based on Group III and one with a high PAO and ester content carry the same designation and are still not the same product.

Mineral oil: where it still belongs today

Purely mineral motor oils have become the exception for cars. They are thicker in the cold, evaporate more and age faster. Their place is in older engines with simple requirements, in commercial vehicles and agricultural machinery running monograde oils, in classic cars whose materials were matched to the oils of their day, and anywhere the oil is changed at short intervals. An engine whose manufacturer demands a modern approval is not correctly filled with one.

Semi-synthetic: the blend

Semi-synthetic oils blend mineral base oil with a synthetic share. No standard specifies how large that share is, which makes the term useless as a statement of quality. These oils are typical in the 10W-40 grade, where for decades they were the standard fill for vehicles with medium mileage. For engines whose approval is met by a 10W-40 they remain a sound choice; the available products are listed under 10W-40.

Fully synthetic: why modern engines need it

Low viscosity grades such as 0W-20 or 5W-30 cannot sensibly be made with mineral base oil: the oil would have to contain so many viscosity index improvers that it would quickly lose thickness under shear. Add to that turbochargers with high bearing temperatures, start-stop systems with many start-ups, and change intervals a mineral oil cannot sustain. Fully synthetic oils reach the lubrication points faster in the cold, stay more stable in the heat and evaporate less, which lowers oil consumption. The widest choice is in 5W-30.

Switching on older engines

Switching from mineral to synthetic oil is technically uncritical as long as the manufacturer's approval continues to be met. Two points matter in practice. First, a synthetic oil can loosen deposits that until then were sealing a leak in a heavily sludged engine; any subsequent oil loss is then not down to the new oil but was merely hidden before. Second, a thicker oil is no substitute for a repair: going up a grade to counter high oil consumption masks the symptom and, on modern engines, risks hydraulic systems such as the camshaft adjustment working outside their design range.

What decides in the end

The order when buying is always the same: first the approval prescribed by the manufacturer, then the viscosity grade, then brand and price. The terms on the front of the container come right at the end, because they are not standardised. How to read the information on the back is explained in the article Reading manufacturer approvals and specifications; which oil is approved for your vehicle is shown by the oil finder.