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What's the difference between synthetic and mineral oil?
What is viscosity?
What is a multigrade oil?
What are additives and why are they used?
My oil consumption is very high. Why?
Does the colour of the oil mean anything?
What do the classifications and specifications on the back of the pack mean?
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Synthetic lubricants are made up of molecules that have been modified under complex chemical processes and allow for enhanced performance under extreme conditions of temperature, pressure and forces. Mineral lubricants are composed of molecules present in crude oil that are separated in the distillation process at a refinery.
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| A liquid that has a relatively high resistance to flow can be described as viscous. For example, water has a low viscosity compared with honey, so in this case honey has a higher viscosity than water under the same temperature. A good quality lubricant keeps its viscosity steady under different temperature and usage conditions for a longer period of time.
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Lubricants that are able to maintain their performance in high and low temperatures are called multigrade. They are defined by two numbers. The first (followed by a W) indicates the lubricant’s viscosity under lower temperatures. The second and higher number indicates the lubricant’s viscosity under greater temperatures. A multigrade lubricant minimises viscosity differences under temperature variations.
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| Additives are chemical compounds which, when added to base oils, improve the performance of the lubricants, protecting them from aging and allowing them to respond to all the demands of the modern engine. The blend of various additives is what sets a quality lubricant such as Shell Helix apart from the competition.
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You would be wise to check for faults, taking into account the type of engine, its age and the driving conditions to which it has been exposed. Faults could be caused by a leak in junctions or seals, wear to pistons or cylinders, or deposits limiting the lubrication of some surfaces.
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| The colour of an oil has no special meaning, but depends on a number of factors, such as the refining process and the types of base oils, additives and crude oil used. Oil may turn a darker colour after use because it contains a number of suspension particles – the reason for this is that oil cleans the engine of particles and other deposits produced by the high temperature combustion, keeping the particles in suspension to avoid depositing them on the engine’s surface.
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Lubricants vary in their use and quality. Some are only suitable for particular engines, while others can only be used in certain weather. Even two oils optimised for the same engine and weather conditions can have vastly different levels of performance. Universal criteria to describe oil performance and usage are therefore used. The main classifications boards are:
Society of Automotive Engineers (SAE) - defines oil according to its viscosity. Oil can be monograde or multigrade, depending on its viscosity performance at various temperatures.
American Petroleum Institute (API) - classifies oils according to their capacity to protect the engine. The letter S is used for petrol engines and the letter C for diesel. The letter that follows the S or the C indicates the degree of performance and protection, with A the lowest and X highest. Some oils can be used in petrol and diesel engines, so their classification is stated as SJ/CF or CF-4/SJ.
Association des Constructeurs Europeens d'Automobiles (ACEA) - similar to the API, with the difference that while the API is based on American Engines, the ACEA is based on European engines. The ACEA also uses letters to denote classifications.
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