It’s the number one distinguishing feature on the label of a jug or drum of oil. It’s how we can tell one oil from another. It guides us to the oil we need for our application. But what is viscosity, actually, and why does it matter? We know generally that higher viscosity oils are “thicker” and lower viscosity are “thinner,” but beyond that, what do those numbers really mean? How does oil viscosity affect fuel economy? Why are we seeing a trend toward lower viscosities, and what does that mean for your equipment?

Those are all fair questions. As lubricant experts, we should not take for granted that our customers understand the thinking that goes into formulating different viscosity grades. So let’s break it down.

In simple terms, viscosity is a measure of a fluid’s resistance to flow. Consider water, for example. It flows very easily. It is extremely low in viscosity. Honey, on the other hand, takes a little more effort to make it flow. It is much higher in viscosity compared to water. It’s much the same with engine lubricants.

The main purpose of a lubricant is to control friction and wear. Viscosity gives the lubricant its ability to form a “film” that keeps moving surfaces apart. Among the variables we have to account for when measuring viscosity (i.e. measuring resistance to flow), fluctuation in temperatures is probably the most significant. If you measure viscosity when the fluid is cold, you're going to get a higher number than if you measure the viscosity when the fluid is hot. A fluid when cold will be more viscous than when it is hot. You can measure viscosity over a range of temperatures, and you'll get a curve which indicates the performance of the fluid – meaning its ability to form that protective film – at different temperatures.

Another factor affecting lubricant performance is shear, which refers to the ability of the oil to flow through openings of different widths. Narrow gaps, such as those between bearings or shafts, are what we call a high-shear environment, whereas the oil passages through which oil flows more easily represent a low-shear environment. In a high-shear situation, the oil pump has to work harder to push the oil through those narrow gaps, which drives up fuel consumption. A lower viscosity oil – one that is less resistant to flow – will require less effort from the oil pump, which means less fuel consumption. That is why lower viscosity oils can help deliver better fuel economy.

That brings us to the two numbers used to designate the oil’s viscosity – what do they signify? Let’s use SAE 15W-40 as an example. The first number, 15, is an indicator of how the fluid will flow in cold temperatures. A 15W is suitable for use down to about -4 degrees Fahrenheit, a 10W down to -13^°F, a 5W down to -22^°F and a 0W to about -30^°F, far below zero. In other words, lower numbers like 0W or 5W tell you that the oil is suitable for use in very cold weather.

The second number, 40 in this case, tells you how the oil flows in a hot engine. The higher the number, the thicker the oil film that will be produced. You might think intuitively that “thicker is always better,” especially if your primary objective is engine protection rather than fuel economy. However, a lot of wear happens when you cold-start equipment, and a thinner oil such as a SAE 10W-30 is going to flow through the engine more quickly than a SAE 15W-40 in cold conditions. In other words, you’ll actually experience less wear with a thinner oil when it’s cold.

With its higher temperatures and heavy loads, heavy duty service is much harder on engine oils than a typical passenger car, where we are used to seeing 0W viscosities. However, the engineering of heavy-duty engines is changing. Older equipment was designed for the fuel and the engine oils of its day, but things have changed since 2010. Most heavy-duty engine OEMs in the U.S. now factory fill with SAE 10W-30 oils. Today’s oil pumps are more efficient and more sophisticated. They require less power and can now dynamically match the flow and load to the operating conditions and temperatures. Modern engines are built to operate on lighter oils that help achieve fuel efficiency, and there's really no wear penalty.

We’re now in a transition phase to lower viscosities. Fleets and operators are gradually coming around as they replace their older equipment with newer models. Demand for SAE 15W-40 oils, which are now actually prohibited in some international markets, is likely to decline further, especially when the PC-12 oil categories are introduced with the 2027 model year.

As always, we recommend checking your owner’s manual and OEM recommendations for guidance on the viscosities appropriate to your application and operating conditions. Chevron can also advise you if you’re looking for ways to save on fuel, extend oil drain intervals or reduce oil consumption.