The viscosity refers to how thick or thin the oil is at a given temperature. The Society of Automotive Engineers (SAE) created the grading system, which is generally accepted as the global standard.
The higher a number, the more viscous (thicker) the oil is. For example, if oil has a SAE 10 grade, it will be thinner than SAE 30 oil.
However, newer cars often require ‘multi-grade’ oil that can run under various operating conditions. These oils can run through the engine thinner when running cold and thicker when the engine is hot, creating better performance.
These multi-grade oils are often given a letter designation ‘W’, meaning it was tested in ‘winter’ or colder conditions. So, an oil designated 5W-30 is engineered to act as thin as a SAE 5 grade oil when starting cold, while it will run as an SAE 30 grade oil once the engine is running at its operating temperature.
There are generally two classification systems that are widely used, one from the American Petroleum Institute (API) and another from the Association des Constructeurs Europeens d’Automobiles (ACEA) that is gaining traction as cars become more global.
API classification is generally widespread in the US (and Australia), while ACEA classification is just becoming more common and was developed to recognise the differences between engines developed in the two continents.
API uses a two-letter rating system, starting with ‘S’ for petrol engines and ‘C’ for diesel engines. The second letter refers to the quality of the oil, with the further along it is the higher the quality. Various letters get superseded over the years, for example, the most current standard for petrol engines is SN, with SJ, SL and SM class oils available for older cars.
ACEA classified oils use a letter-number designation, with A, B, C and E before a specific number that relates to a particular type of petrol or diesel engine. Generally speaking, higher quality synthetic or part synthetic oils meet ACEA classification standards.
Generally, the owner’s manual in your car will specify what oil you need for optimal performance (or give you several options), referring to a multi-grade oil required. It is important to refer to that before purchasing any oil, as putting the wrong fluids in can damage your entire engine.
If you’re still unsure as to how to pick an oil, speak to a service advisor at Motorama who can go through everything from the oil filter to the dipstick so your engine is running as smooth as possible.