When it comes to discussions about oil, there are several topics that seem to get talked about. One is about change intervals, another is about best brands, and yet another involves what oil viscosity is best for a given application. You would think the last has an easy answer, as the manufacturer of your truck gives you a recommendation. However, there is still a lot of confusion on the subject.

If you are one of the many looking for answers, we found the short video above from AMSOIL that talks about oil viscosity and what it all means to you. But, before you check out the video, we have a little information you might find useful.

Surely, we all know the physical properties of lubricants are measured by viscosity, shear stability, high- and low-temperature performance, water resistance, and volatility. A quality oil must maintain each of those properties in order to optimize a lubricant’s performance. To do this, oil manufacturers must manage these properties by using different base oils and additives. Controlling a lubricant’s viscosity and how it changes under different temperatures and operating conditions is one of the more important properties that determine a lubricant’s performance and protection capability.

Scientifically speaking, external forces such as gravity act upon everything. When discussing fluids, gravity causes the molecules within the fluid to move against each other. This results in friction at a molecular level that resists flow. The higher the viscosity, the greater the internal friction. How the viscosity of a fluid reacts to changes in temperature and pressure determines how well a fluid can perform the basic functions of a lubricant.

The base oils used in lubricating oils thicken as they cool and will even solidify if the temperature drops below a certain threshold, called the pour point. This thickening will increase the lubricant’s load-carrying capabilities but will impair its ability to circulate. On the other hand, lubricants will get thinner as they get heated, which decreases their ability to carry a load and prevent metal-to-metal contact.

Extreme pressure is another factor that can lower viscosity. This is called mechanical shear, and it decreases the lubricant’s film strength and its ability to prevent metal-to-metal contact and wear. Some enthusiasts choose to utilize a higher-viscosity oil to combat this, but choosing a higher-viscosity oil can cause oil starvation. This is because the thicker oil will not flow freely through smaller passages. Proper viscosity for the intended application is critical to ensure oil circulates properly and provides adequate film strength under pressure.

A lubricant’s formulation and quality will impact how much its viscosity will decrease with an increase in temperature. The change in oil viscosity due to a temperature change is called the viscosity index (VI). Understanding VI is important to discern whether the lubricant in question meets the requirements of your application based on the operating temperature range. The Viscosity Index of oil refers to how much the oil’s viscosity changes due to temperature. A high VI (above 95) indicates the fluid undergoes little viscosity change due to temperature fluctuations, while a low VI indicates a relatively large viscosity change.

In case you care, to determine an oil’s VI, the viscosity needs to be measured at two temperatures: 104 degrees Fahrenheit and 212 degrees Fahrenheit. The results are then compared to a scale based on two reference oils. While the VI has no actual unit of measure, it is well known that the viscosity index of a conventional mineral oil is between 95 and 100. A highly refined mineral oil has a VI of approximately 120. AMSOIL synthetic oils have a very high VI number, which means the lubricant changes viscosity at a lower rate based on the temperature. In other words, it is a very stable oil.

But that’s enough with the scientific reading. Take a few minutes to check out the video above to learn a few things about oil viscosity and how it affects the way an oil can protect your engine.