However, it is also known that temperature impacts the viscosity. It is well-known from Hydrodynamic Lubrication Theory that viscosity plays a central role in the lubrication regime encountered in the machine element – the higher is the viscosity, the thicker is the oil film that separates the surfaces from touching each other. Test your oil while figuring out what to use.Viscosity is a measure of a lubricating oil’s resistant to flow. The key is to find out why and repair your engine or adjust your driving habits accordingly, and to correct the viscosity and optimize your engine’s efficiency. When your oil’s viscosity comes back as either lower or higher than the “Should Be” range, something is causing it. Some engines will shear the viscosity down no matter what oil you use. Exposure to excessive heat (leaving the oil in place too long, engine overheating) can increase the viscosity of engine oil, though leaving ATF in place too long can cause it to get thinner, not thicker. Excessive soot and antifreeze often increase an oil’s viscosity. Moisture and fuel can change the viscosity, depending on the contaminant and how long it has been present in the oil. Another thing that can change a viscosity is contamination. Adding anything foreign to your oil can change its viscosity - some types of aftermarket additives cause a high viscosity, and some solvent-type additives can cause the viscosity to thin out.
Feel free to try different grades until you find one that suits your particular situation. Many manufacturers use 5W/20 or 0W/20 oil at the factory (even in trucks) and recommend it for everyday use for many light vehicles. The common 10W/30 of a decade or two ago has become a 5W/30, 5W/20, or 0W/20. Over the last few years we have seen a trend of lighter oil for new engines. If you’re racing, for example, that may require a thicker oil to stand up to the heat demands of more extreme use. For the sake of efficiency, you want to run the lightest grade oil in your engine possible, within limits. Going to another viscosity is an experiment, but it’s usually a harmless one. Engine manufacturers dyno-test their engines using a specific viscosity oil, so when you use the viscosity they recommend, you are working with a known result. People often ask us if it’s okay to use a different viscosity oil than what the manufacturer recommends.
The VI additives in multi-grade oil help it move more easily through a cold engine upon start-up, but still provide cushion and lubrication when it’s hot. That’s where viscosity improvers enter the picture. But we want it to act like it has a narrow viscosity range, like water, maintaining a fairly consistent flow rate regardless of whether the oil is cold or warm. When it comes to engine oil, it naturally has a wide viscosity range, like honey, flowing slowly when it’s cold and faster when it’s hot. That’s because water has a very narrow viscosity range, whereas honey’s is much wider. Water, on the other hand, flows at pretty much the same rate whether it’s hot or cold. Cold honey flows very slowly, but if you put it in the microwave and heat it up, it will flow much more easily. Think of the difference between honey and water. A major difference between the two is simply the addition of a VI additive, which allows the oil to maintain more or less the same flow rate regardless of its operating environment. (Download your own viscosity chart here.) No matter what you call it, the number given defines the thickness of the oil at the standard high temperature.Įngine oil can be either straight weight or a multi-grade viscosity.
In the same respect, there are many ways to describe viscosity: engines use the SAE engine chart, industrial equipment mostly uses the ISO chart, gear oils use the SAE gear chart, etc. We can do the low-temp viscosity too, if you’re interested, but the engine spends so little time running at the low-temp viscosity that it’s not a useful test for most people.Īn apple is an apple, no matter what language you use to describe it. At Blackstone we report the high-temp viscosity, which is generally the temperature the engine is at while it’s running and the temperature at which the oil spends most of its time. The viscosity of an oil could be reported at any temperature, but to standardize things, most laboratories report either a low temp (100F or 40C) or a high temp viscosity (212F or 100C) and stick with either Fahrenheit or Celsius. To be more specific, it is the thickness of oil at a given temperature. Commonly, though, we think of it as an oil’s thickness. Technically, viscosity is defined as resistance to flow.
We tend to choose an oil with a viscosity that we believe is correct for our particular engine, but would another viscosity improve or reduce the life of the engine? Can we pick and choose a viscosity outside the manufacturer’s recommendations? Most of us have only a vague understanding of viscosity.