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A methodology for measuring oil viscosity in an internal combustion engine has been developed that is based on measured values of oil pressure and oil temperature at a relatively low engine speed near idle. Engine oil pressure results from the resistance of the oil to flow under the pumping action of the oil pump. The resistance to flow, in turn, is a function of both the viscosity of the oil and the flow rate. At a constant oil flow rate, a higher oil viscosity will result in a higher oil pressure. Oil viscosity is an important factor in determining the ability of the oil to provide effective lubrication and, for example, can be used as an indicator of the need to change the oil. This report describes the operational principles of the methodology for determining engine oil viscosity and a proof of concept based on a simple vehicle test.