Lubricants are often subjected to mechanical shearing, for example in automotive engines. This shearing causes lubricants to lose their viscosity. When subjected to sufficiently large number of cycles of shear, viscosity may drop below acceptable levels and may call of replacement failing which surfaces may become prone to higher wear. Even lubricants that have consistent viscosity through a wide temperature range (high viscosity index) may still be susceptible to loss of viscosity when subjected the mechanical shearing.

Shear Stability Tester is an instrument used to determine an oils ability to retain its viscosity over a number of cycles of shear. The test oil is first measured for its initial viscosity after which it is subjected to shearing in intervals of 30 cycles. Viscosity is measured and compared to the oils initial viscosity following 30, 90 and 120 cycles. Oils that show a lower drop in viscosity over the duration of test are said to have better shear stability than oils that exhibit a relatively higher drop in viscosity.