![]() 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. ![]()
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