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If you have ever gone into your local Bursons Auto Parts shop and looked for oil for your vehicle you will see a vast array of engine oil available in different brands and grades, usually there is a chart that will tell you what you require.motor oils on shelf

So what are the numbers all about?Motor oil bottle

To understand the numbers we need to understand the term Viscosity

Picture a jar of honey, when it is cold it is thicker and slow to pour allowing it to stick to surfaces.

When it is hot it can become almost a water consistency allowing it to flow freely over surfaces.

The engine oil in your vehicle needs to do the same, when the vehicle is cold we want a certain amount of oil to adhere to the various internal parts of the engine, to prevent wear on start up.

As the engine heats up we need the oil to become thinner so it can still keep the parts lubricated but can flow freely around the engine and its various components.

 So “Viscosity, is a fluids resistance to flow”

The higher the number the more resistant the oil is to thinning

Now let’s look at those numbers


The first number 10 is its viscosity or thickness at cold temperatures (usually measured at zero degrees Fahrenheit)

The W usually stands for winter indicating it is suitable for Winter or Summer use

And the second number 40 after the dash - indicates its viscosity or thickness at hot temperatures (usually measured at 212 degrees Fahrenheit)

So a 10W-40 grade oil is more resistant to thinning than a 10W-30 grade oil.

There are lots of different types of oils on the market and you must select the oil grade and purpose recommended by the manufacturer.

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