Explanations of technical and specification references can be accessed by clicking on the relevant buttons to the left of this page.
API Specifications - API stands for American Petroleum Institute.
The American Petroleum Institute (API) engine oil classification system was set up as a joint effort by API, ASTM (American Society for Testing and Materials) and SAE (Society of Automotive Engineers). The letter classification system is a method of classifying engine oils according to their performance characteristics, and relating this to their intended type of service.
The classification of a given oil is unrelated in the main, to oil viscosity.
American Petroleum Institute service classifications are a two-letter rating beginning with “S” for petrol engine oils and “C” for diesel engine oils. The second letter designates the oil’s quality standard, beginning with the letter “A”. The lower down the alphabet of the second letter, the higher the oil’s quality. Many oils meet standards for both petrol and diesel engines and will be marked with a dual service classification, for example SJ/CF.
ACEA Specifications - ACEA stands for the Association des Constructeurs Europeens d’Automobiles, the Association of European Automobile Manufacturers.
Like the API service classifications, used in the US and Australia for many years, the European ACEA system is the accepted standard in Europe.
ACEA standards recognise that European engines differ from US (style) engines in both their design and operating conditions and that the demands on their oils are also different. This requires the oils used in European engines to be subject to a separate and unique classification system. For this reason it is difficult to compare the common API classification and ACEA standards, as the test sequences for them are quite different.
Engine oils usually carry both API and ACEA qualifying classifications.
Viscosity Information - Viscosity is the oil's resistance to flow or, an oil's speed of flow as measured through a device known as a viscometer. The thicker (higher viscosity) an oil, the slower it will flow.
Oil with a higher viscosity can stand greater pressure without being squeezed out of the lubricating surfaces. However, the high internal friction of the oil may offer greater resistance to the movement of the lubricating parts. An oil of lower viscosity offers less resistance to the moving parts but the oil can be easily squeezed out of the lubricating surfaces. It is therefore important to select a lubricating oil of appropriate viscosity to achieve optimum lubrication effect.
SAE viscosity ratings - SAE (Society of Automotive Engineers) ratings designate the oil’s viscosity; simply put, how thick or thin an oil is at a certain temperature. For example SAE 30 oil is “thinner” (less viscous) than SAE 50 oil.
The vast majority of carmakers specify multi-grade engine oils which are designed to act like a thin oil when cold so that they can circulate through the engine quickly on start up, and like a thicker oil when hot, to provide the necessary engine protection. An example of a multi grade oil designation is 5W-30. The 5W (W = winter) indicates how the oil would behave when cold, while the 30 is how it acts when hot.
Viscosity Index (VI) - is an indication of how the viscosity of a liquid varies with temperature. A high VI means the liquid does not thin out as much when temperature rises and is more thermally stable.