Viscosity is defined as a measurement of resistance to flow and is a key physical property of lubricants. Industrial lubricating oils are generally measured at 40° C and results are reported as centistokes (cSt). Engine oils are most often measured at 100° C. Most industrial lubricating oils are classified by their viscosity. For example, an ISO Viscosity Grade 32 (ISO VG) will have a viscosity of 32 cSt @ 40° C, ± 10%. Engine oils are classified by their Society of Automotive Engineers (SAE) grade. SAE 20 or SAE 30 for example. Engine oils often have additives that give them multi-grade characteristics, such as SAE 10W40. This refers to the oil that has the viscosity characteristics of an SAE 10 oil when it is cold, and the viscosity characteristics of a SAE 40 weight oil in normal temperatures. Viscosity of oils is inversely proportional to temperature.
Viscosity is determined by measuring the time it takes for a liquid to flow between two sensors on a glass tube immersed in a constant temperature bath. The fluid is allowed to reach bath temperature (usually 40° C) before the analysis is performed. The tube is calibrated using NIST Traceable viscosity standards.
Viscosity increases are normally a result of lubricant oxidation and degradation or contamination with a higher grade oil.
Viscosity decreases are almost always a result of contamination, either with fuel in the case of engine oils, or product in the case of industrial oils.
Viscosity Index is a calculated number that indicates the rate of viscosity change as the lubricant is heated. The less the change, the higher the Viscosity Index number. The lubricant’s viscosity is determined at two different temperatures, usually 40° C and 100° C, and using an API formula, the Viscosity Index is calculated. Viscosity indexes of 95 to 105 are normal for most industrial mineral oils. Severely hydro-cracked base stock and PolyAlphaOlefin (PAO) synthetic base stock have much higher Viscosity Indexes, usually in the 120’s to 140’s. Diester synthetic base stock on the other hand has a low Viscosity Index, around 76.