Info Centre >> Fuel and Lube Oil Technical Manual >> 7. Lube Oil - Test Results >> 7.1 Key Condition Indicators for Lube Oils >> 7.1.1 Viscosity
Viscosity is the most important property of the oil. Oil of the correct viscosity will provide optimum film strength in bearing clearance with minimum friction losses and leakage.
Low temperature measurements of shear are taken for multigrade oils and the SAE number is quoted as a double grade, i.e. 15W/40, where the first number relates to the viscosity in centiPoises (cP) at a reduced temperature. High temperature measurements of shear are also required to be met by oils claiming their SAE viscosity grades. Generally, hydraulic, turbine and gear oils conform to ISO viscosity designations that are listed right.
The viscosity of engine oil may fall due to distillate fuel oil dilution, by topping up with an incorrect oil grade, or shear of polymer additives. Viscosity may increase due to excessive soot loading (Insolubles content) or if the centrifuge or filters are not operating correctly. For all oils, ageing caused by oxidation and thermal degradation may lead to thickening and an increase in viscosity. Viscosity must always be referred to as a standard temperature usually 40°C or 100°C.
