Viscosity And Engine Oil 101

Understanding viscosity is the first step in choosing the proper motor oil for your engine’s needs. Choosing the wrong motor oil can lead to leaking seals and reduced energy efficiency. In this article, we will cover the basic principles of viscosity and engine oil, the effects of viscosity on your engine, and how to choose a motor oil that is right for you.

Understanding Viscosity

Viscosity is a term used to describe the physical property of a fluid, most often referring to a fluid’s thickness. Understanding a fluid’s thickness helps measure the fluid’s resistance to flow. For example, when describing the viscosity of honey and water, the thicker the fluid, the higher the viscosity. Honey would be high in viscosity and water would be low. To prove this theory, pour honey and water in two separate cups. When you flip both cups upside down, you will notice that the rate in which honey runs down the cup is slower in comparison to water.

Honey is high in viscosity due to its thickness

Viscosity is affected by temperature. At higher temperatures, viscosity decreases (thins), where as in lower temperatures viscosity increases (thickens). By adding heat to liquids, the molecules within begin to move and allows the liquid to become more fluid, resulting in a decrease of viscosity.  Consider syrup for waffles. When you first  pour your syrup, the viscosity is high when cold. When heated in the microwave, viscosity decreases and allows the syrup to flow more freely.

Viscosity And Engine Oil

As a motorist, your goal should be to get the most out of your fuel, however, an excess of friction can cause energy to be lost and bad performance beneath your hood. Friction is the force that resists relative motion between two bodies in contact. Without proper lubrication, the parts within your engine run against each other at a rapid speed causing friction and heat. Worn parts destroy your gas mileage and emissions because your engine is pushed to work harder. A good lubricant will protect your motor from wear and reduce temperatures, increasing energy output.

Engine oil viscosity describes your oil’s ability to pour at specific temperatures. As illustrated above, thin oils will have a lower viscosity and pour easier at low temperatures in comparison to thicker oils that will have high viscosities and require higher temperatures to increase flow. Engine oil viscosity has dual roles. Thin oils reduce friction in engines and help start engines faster in cold weather. Thick oils are better at maintaining film strength and oil pressure at high temperatures and loads.

Measuring Engine Oil Viscosity

To help consumers quickly identify the viscosity of oil, The Society of Automotive Engineers developed a scale for engine oils (motor grades) and transmission fluids.

Viscosity is illustrated using the classification XW-XX. The number before “W” rates the oil’s viscosity (it’s flow) at zero degrees Fahrenheit. The “W” is abbreviated for the word “winter”. This means that the number preceding “W” is an indication of a fluid’s lower viscosity in winter versus the higher number which is the oil’s operating temperature. In summary, the lower the number before “W”, the less the oil thickens in cold weather.

The numbers following “XW” represent the viscosity at 100 degrees Celsius and indicates the oil’s resistance to thinning at higher temperatures.

The OW-20 and the 5W-20 on the Havoline Pro DS and the Havoline Synthetic Blend products indicate each oil’s viscosity.

How This Affects Your Engine

How well a lubricant’s viscosity reacts to changes in temperature, pressure, or speed will determine how well the oil will protect your vehicle. If your vehicle’s lubricant viscosity is too low for your engine, this may cause:

  1. Increased metal to metal contact, resulting in increased friction and wear
  2. Increased oil consumption
  3. Leaking seals

If your vehicle’s lubricant viscosity is too high, you could cause damage to your engine by:

  1. Increased fluid friction
  2. Poor starting temperature at cold weather
  3. Reduced energy efficiency

How To Choose The Lubricant With The Proper Viscosity For Your Vehicle

  • Always start with your owner’s manual. The vehicle manufacturer will specify which motor oil is best for your engine.
  • You will always want a lubricant that resists thickening when cold, but protects against wear when hot.
  • Often, synthetic lubricants offer better cold flow when the temperature drops and improved protection once your engine has reached operating temperature.

Brewer-Hendley Oil Company’s extensive line of Chevron motor oils provides unbeatable, nonstop protection to keep today’s modern engines going. For more information Visit our website at

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