Oil grades and classes | SAE | Explained

This is very clear that in order for the oil to do its job of separating two moving objects, there must be a space between them that the oil will fill. Nevertheless, this means that if the oil were completely fluid it would easily escape through the gap, so a certain viscosity or’ thickness’ is required for an oil to be effective.

This seems to suggest that a’ thick,’ viscous oil is better than a’ thin’ oil, but this is obviously not the case: viscosity is an indicator of the friction between the oil’s molecules themselves, so a very viscous oil makes it difficult to start, wastes power and generates extra heat.

To put it another way, you need to select the right viscosity oil for your engine. There are several different ways to quantify viscosity, but to make life fairly easy, oils are now graded according to a number system developed by the American Society of Automotive Engineers , where the higher the number, the thicker the oil. Your engine manual may specify, for instance, that it needs an oil grade ‘SAE 40’.

The image is made somewhat more complicated by the fact that oils are less viscous when they warm up, so that an oil that is right at normal operating temperature can be too thick to start easily in the winters.

To overcome this, using a much’ lighter’ oil in winter was once standard practice, and recognizing increased wear as a penalty that had to be paid. SAE catered for this by introducing a second series of ‘Winter’ grades, such as SAE 10W.

Oil technology has made considerable progress since the implementation of the SAE grades. Today, oil-mixed additives make it much less susceptible to temperature changes. It means that most modern engine oils can be used in both summer and winter, and thus SAE grades are seen together in summer and winter, such as SAE 20W/50 or SAE 15W/40.

Also’ regular’ engine oils nowadays contain a mixture of other additives that tend to improve their performance in particular aspects. Inevitably this means that certain oils are’ better’ or’ worse’ than others, and various organizations have introduced performance criteria to classify oils appropriate for particular job.

To understand a SAE grade let us take example of SAE 15W/40, here 15W indicated the grade of oil for winters and 40 indicates oil grade during summers.