Viscosity – This is defined as resistance to flow, or how thick an oil is.It changes with temperature i.e. as you heat an oil it will begin to thin and as the oil is cooled it will begin to thicken. The importance of viscosity is that at high temperatures the lubricant film must be thick enough to separate the two surfaces, but thin enough to still be energy efficient. Many diesel engines are produced with a lower viscosity base oils with special Viscosity Index Improver additives so at low temperatures they are fuel efficient in cranking the engine, but at high operating temperatures still give sufficient film thickness to prevent wear. These oils are called multi-grades (e.g. 15W40, 5W30 etc). Viscosity is measured at two temperatures.
- Viscosity at 40’C – All lubricating fluids (as standard) are measured at this temperature and is the standard temperature to measure fluids for industrial applications with ISO Viscosity grade ratings.
- Viscosity at 100’C – This is performed on all multi-grade (denoted with a “W” in the grade, where W = Winter) and SAE grade fluids used in engines and automotive gear oils.
|Viscosity||Common Causes||Potential Result||Potential Advice|
Download our viscosity classification handout here: LearnOilAnalysis Viscosity Grade Chart.
Find out how viscosity is tested below, what is it’s significance and how to diagnose viscosity changes. Then test what you learn in our test your knowledge quiz below.
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What is the most important property of an oil?Correct
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What are two most common temperatures to measure viscosity?Correct
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Which of the following is not an ISO Viscosity grade?Correct
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Which statement regarding viscosity specifications is most true?Correct
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An ISO grade is said to be within specification for a new oil when…Correct
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Viscosity Index measures…Correct
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What are likely sources of increased viscosity?Correct