![]() Also think how much harder it would be in corn syrup than it would be in water. But as you try to throw it faster, the force necessary goes up at a higher rate than the change in speed of your arm (dynamic). If you go through the motion very slowly, the amount of work isn’t much different than doing it in air (kinematic). Think about throwing a baseball under water. Thicker makes it consume more energy just to move. The pumping difference is pretty straightforward, but what about shearing oil? That’s the hydrodynamic lubrication created when the crank slides across the wedge of oil provided by your oil pump inside the bearings. If the engine is designed and manufactured to use a 10W-30, it can improve fuel economy and power over using a 15W-40. ![]() This is why many heavy-duty oils are moving from the traditional 15W-40 to 10W-30. Nice mental image now? Lower viscosity is easier to pump and to shear. Parasitic losses are things that tax the engine’s power, think parasite, like a leach. The tradeoff is choosing between lower viscosity to decrease pumping and shearing losses, versus keeping the crankshaft from touching the bushings by floating on a wedge of oil (the technical term is hydrodynamic lubrication.) Lower viscosity oil reduces parasitic losses, improving fuel economy and power. We have the technology in oil today to modify the viscosity such that the viscosity acts differently as speed changes, to help us meet the requirements of today’s modern engines. The common dynamic viscosity used for fuel economy improvements is referred to as the High Temperature High Shear (HTHS) viscosity. Two at low temperature, Cold Cranking Viscosity, CCS, done at high speed (shear) and Mini Rotary Viscosity, MRV, one at low speed (shear). There are three dynamic viscosity measurements that define the SAE grade. The kinetic viscosity is measured at a very slow speed, under the influence of only gravity. That means the biggest variable is speed. The temperature variation goes from ambient to normal engine operating temperature which is usually around 100 degrees C. Mostly we are working at atmospheric pressure or a bit above so we can consider that relatively constant. How is it that CK oils are different viscosity than FA oils, even if they are both 10W-30? Viscosity is affected by three things, temperature, pressure, and the speed that you shear it (which relates to engine speed). Now let’s talk about the newest API rating. Just remember, corn syrup is high, corn liquor is low. However, when you compare a 10W-30 vs a 15W-40, at both low and high temperatures the 10W-30 will be thinner, with less resistance to flow. For example, a 5W-40 oil will be thinner in cold temperatures than a 15W-40, but at normal operating temperatures both oils will flow the same. The lower the kinematic viscosity number, the thinner the oil. The second number to the right (40 in this example) represents the kinematic viscosity at a normal engine operating temperature, usually 100 degrees C. ![]() In 15W-40, the first number on the left (15 here) represents the cold temperature viscosity and contains the letter “W” (which stands for winter, engineers are so creative). Since most heavy-duty oil is multi viscosity, we will concentrate on the two-number system. A kinematic viscosity grade typically consists of either a single number (mono viscosity grade) or two numbers with a dash between them (multi viscosity grade). Kinematic viscosity is reported in the numbers we are all familiar with on a container of oil. Starting with the basics, there are two types of viscosity, kinematic and dynamic. Let’s dig in so this is not just a coin flip selection. If your engine has been modified, then understanding this can help you make the right choice. Most engines will allow various viscosity grades, and that allows you to make the selection that best fits the application of your equipment. If your engine is stock, stick to what the engineers were using when they designed and developed your engine. The first thing to consider is the OEM recommended viscosity grades. There are many different viscosity grades available, and thinner motor oils are becoming more common.
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