What Is the Difference Between ISO Grades of Oil?

automobileglobe.comAll Industrial machines and even hand tools depend on lubricants, or oils, to keep function properly. This material makes sure that parts can move freely without causing damage. Hydraulics basically used mineral oil-based fluids to send power or heat to elements of various machinery, excavators inclusive. A perhaps more common use of hydraulic oils is that which automobiles use for braking (brake fluid). This fluid is one of many to which the ISO viscosity gradient scale can be put to use.


The International Standards Organization Viscosity Grade, or ISO VG, is a numerical rating of the viscosity of oils and lubricants as made by several organizations in 1975. The International Standards Organization (ISO), American Society for Testing and Materials (ASTM), Society for Tribologists and Lubrication Engineers (STLE), British Standards Institute (BSI), and Deutsches Institute for Normung (DIN) made the ISO VG to help standardize the industry. Lubricant and oil supplies and manufacturers, as well as manufacturers of machinery that make use of lubricant, use this ranking in their work as it describes the material’s resistance to flow.

What is its Significance

As the viscosity of the oil adds up, so does the density of the material, as a higher density results in oil that is less likely to respond to flow or other movement. Thus, an oil or lubricant with a viscosity grade of 220 is thicker and more solid-like than an oil with a VG of 100 or 68. The grade is a literal measurement of the oil’s ratio of absolute viscosity in centipoise (a unit of measurement) to the density, equally known as centistoke.


Since its establishment in 1975, the organizations have developed 20 viscosity gradients to cover the range of oils and lubricants that are very common in hydraulic application. The lowest common ISO grade is 32 and the scale ranges up to 220. The scale also includes grades 46, 68, 100 and 150.


Since the viscosity of oil and other liquids is dependent upon temperature, the ISO grade is basically applicable at a specific temperature. Base ISO grades are calculated when the oil is at a temperature of 40 degrees C (104 degrees F) and raising or lowering the temperature of the material will affect the oil’s resistance to movement such as flow. For example, raising the temperature to 100 degrees Celsius will vary the number of centistokes from a grade to just 5.4 centistokes, in comparison to the 32 centistokes at 40-degrees Celsius. At this kind of temperature, the oil is more likely to be affected by flow.

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