What is oil viscosity?

Viscosity is an important criterion  of any lubricating oil. It is a measure of a fluid’s thickness or resistance to flow. For example, honey is thick and water is thin, so honey has a higher viscosity than water. Oil viscosity needs to suit the right ambient temperatures. If it’s too thick when the engine is cold, it won’t move around the engine. And if it becomes too thin when the engine is hot, it won’t give the right protection to the engine parts.

Optimising an oil’s viscosity, or thickness, helps maximise energy efficiency while avoiding component wear. Viscosity modifiers increase the viscosity of your oil at high temperature but have little effect on low-temperature viscosity. These enable your oil to flow properly when cold and also to remain thick enough to protect your engine components at high temperatures.

Lower-viscosity grades of oil such as Shell Helix Ultra, make it easier for your engine to start from cold because they present less resistance to moving parts and hence take less power from your engine. This also means that you get enhanced fuel economy. To find the right oil viscosity for your engine, check your manufacturer’s guidelines and bear in mind the weather conditions you live and drive in.

What do the oil numbers mean?

Multigrade oils are those oils that have two numbers on the grade, indicating that the oil is able to maintain engine performance in high and low temperatures. A multigrade lubricant minimises viscosity differences under temperature variations.

The first number on a multigrade oil is normally followed by a W, which stands for winter. This number represents the lubricant’s viscosity under lower temperatures, giving an indication of how the oil will flow in the winter. The lower the first number, the thinner it is at low temperatures.

The second number, which appears after the W, represents the oil’s viscosity under high temperatures.  The higher the number, the thicker the oil will stay at high temperatures. Using the correct viscosity for your engine increases engine performance, reduces engine wear and increases fuel efficiency.

In addition, synthetic oils usually have better low and high temperature properties than normal mineral oils.  This maintains protection while allowing better fuel economy and cold starting.

What’s the difference between mineral and synthetic?

There are two main types of base oil: mineral and fully synthetic. Motor oils labelled as ‘part-’ or ‘semi-synthetic’ or ‘synthetic technology’ contain a mixture of these two types.

Mineral base oils come from crude oil that is processed in a refinery to separate out the fractions with useful lubrication properties and remove unwanted components such as waxes. Unlike some motor oil manufacturers, Shell makes its own top-class mineral base oils by selecting the best crudes and employing stringent quality control procedures throughout the entire production process.

Synthetic base oils give enhanced performance because they are manufactured using a more expensive chemical process than that used in mineral oil production. Because of this, fully synthetic oils like Shell Helix Ultra flow more easily at start-up temperatures, when most wear occurs. They are also more resistant to heat and are more easily protected by antioxidant additives (oxidation is a natural degradation process that occurs in oil over time). Plus they are less volatile than mineral oils.

What are performance additives?

Performance additives give extra protection to the base oil and enable it to protect and clean your engine, helping it to work harder for longer. They come in several kinds that must be carefully selected and blended to give top-class performance:

  • Detergents keep your engine clean and neutralise the corrosive acids that form as fuel is burned.
  • Dispersants remove soot and sludge and hold it in the oil to prevent blockages. This dirt can then be removed from your engine at the next oil change.
  • Anti-wear additives protect your engine by forming a chemical layer between the moving parts. These additives are particularly important when motoring with high loads or during engine start-up.
  • Antioxidants help to delay the natural degradation of your motor oil, hence protecting your engine more effectively for longer.
  • Friction modifiers reduce the drag between moving parts to increase fuel economy.
  • Anti-rust additives are essential elements that prevent engine corrosion.