Oil is pumped through a Diesel Injector nozzle under certain flow and pressues and permant vis loss is recorded.
1.1 These test methods measure the percent viscosity loss at 100°C of polymer-containing fluids when evaluated by either of two diesel injector apparatus procedures. Procedure A uses European Diesel Injector Test Equipment and Procedure B uses Fuel Injector Shear Stability Test (FISST) equipment. The viscosity loss reflects polymer degradation due to shear at the nozzle.
I do not run or have the have full ASTM test protocall avalible but this gives idea.