Crude oil is refined and processed to create products with specific characteristics and uses. A key characteristic of refined oil is its viscosity, or ability to flow. This determines its lubricating abilities such as friction reduction and ability to cling to bearings. Drivers may be familiar with the SAE (Society of Automotive Engineers) scale used to describe engine oil, but viscosity is also measured in other units including centistokes (cSt) and Saybolt Universal Seconds (SUS).
Establish the temperature at which the oil was tested to obtain the cSt value. This will usually be either 100 degrees Fahrenheit or 210 degrees Fahrenheit. That is 38 degrees Celsius or 99 degrees Celsius for metric calculations.
Multiply the cSt value by 4.632 if the test temperature was 100 F. The result is the cSt value converted to SUS. For example:
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100 cSt tested at 100 F is equivalent to 463.2 SUS because 100 x 4.632 = 463.2.
Multiply the cSt value by 4.664 if the test temperature was 210 F. The result is the cSt value converted to SUS. For example:
100 cSt tested at 210 F is equivalent to 466.4 SUS because 100 x 4.664 = 466.4.
Viscosity varies with temperature and pressure. Standard calculations assume that the universally agreed temperatures and pressures are used. Under other conditions more complex calculations are required.