AbstractThe relative resistance of VI improvers to breakdown depends on internal structural parameters (chemical type, ratio of monomers, degree of branching) as well as on external factors (test type, duration, severity).Of the many bench tests available to screen shear stability, the 30 cycle CEC injector test is most commonly used for engine oils. Test duration, however, can significantly change the relative ranking of VI improver shear stability. While some polymers stabilise quickly, others continue to degrade when test duration is extended. Some polymers are also more susceptible to the severity of the shearing conditions encountered, e.g. in the FZG and the Tapered Bearing tests.It is therefore important to determine which test, and which set of conditions, will rank shear stability of different VI improver types in the same order as does the field.