A measure of an oil’s irreversible viscosity loss attributable to the viscosity modifier when the oil is subjected to engine operation or special test conditions. Also sometimes called Permanent Shear Stability Index (PSSI), SSI is defined by the equation: SSI = 100(Vo-Vs)/(Vo-Vb), where Vo = viscosity of the unsheared oil, Vs = viscosity of the sheared oil, and Vb = viscosity of the base oil.