Permanent Shear Stability Index (PSSI) is a measure of the loss of viscosity, due to shearing, contributed by a specified additive.

Note 2—For example, a PSSI of 50 means the additive will lose 50 % of the viscosity it contributes to the finished oil.

The selection of appropriate base fluids and additive concentrations to be used in test oils is left to individual operators or companies. These choices will depend on the intended application for the additive.

Note 3—PSSI may depend more strongly on base fluid, additive concentration, additive chemistry, and the presence of other additives for base fluids of unusual composition (for example, esters) or if additives outside the common range of chemistries and concentrations are used. Caution should be exercised when interpreting results from different sources.

1.1 This practice specifies the procedure for the calculation of Permanent Shear Stability Index (PSSI) of an additive using viscosities before and after a shearing procedure.

1.2 PSSI is calculated for a single blend component and can then be used to estimate the effects of that component on finished lubricant blends.

1.3 This practice is applicable to many products and may use data from many different test methods. The calculation is presented in its most general form in order not to restrict its use.