What is oil viscosity?

We are very often asked the question ‘What does oil viscosity mean?’

Oil viscosity is the measure of the “thickness” of a fluid and its resistance to flow, and it is an important characteristic of a lubricant.  The oil viscosity determines the film thickness and film strength, both of which are critical for reducing friction and keeping metal surfaces apart.

The oil viscosity (thickness) at low and high temperatures is measured on a scale determined by the Society of Automotive Engineers and given an SAE number.  Therefore for monograde oils, an SAE 30 oil will be thinner than an SAE 40.

For a multi-grade oil, there are two oil viscosity numbers. The first number precedes the letter ‘W’ which stands for winter.  This number defines how an oil will flow when it is cold, for example when the engine is first started up.   The second number, after the ‘W’, defines how the oil will flow once the engine has warmed up.

Therefore, a 0W30 oil will flow better than a 10W30 when the engine is first started and a 10W30 will flow more easily than a 10W40 when the engine is warm at its normal operating temperature. 

Engine oils become thicker as they cool down and thinner as they heat up.  Thinner, lower viscosity lubricants will provide better protection to engine components when the engine is cold. Whereas, thicker, higher viscosity lubricants will provide a better film strength to protect engine components when the engine is warm.

However, high viscosity or thicker oils are not necessarily better, because if the oil viscosity is too high it can cause excessive energy consumption, heat generation and poor lubrication on start-up when the engine is cold.  If an oil viscosity is too low or too thin, there will be oil film failure and insufficient lubrication, which then leads to friction and wear as well as sensitivity to particle contamination.

It’s important to select the right oil viscosity for your vehicle or application – so check the manufacturers specification (OEM specification) or use our Which Oil? checker to help you find the correct oil for your vehicle.  If you need help on find the right oil, please contact us as our Technical Team are here to offer advice.

For details on all of engine oils produced by Millers Oils, please see our full range of Classic Engine Oils, Motorsport Engine Oils and Road Car Engine Oils.

For other useful info please refer to our FAQs page.