AbstractFactors affecting the mechanical stability of polyalkylmethacrylate type additives are comparatively examined for different shear degradation test methods and routine engine service, taking into account rheological and size exclusion chromatography data. The shear test methods are assessed regarding the MWD of the degraded additives.The extent of degradation of macromolecular additives and the molecular weight distribution (MWD) of the degraded products depend on the nature and extent of shear stress, average molecular weight, MWD, chemical structure, nature of the base oil and temperature. The shape of the MWD of the sonicated and automotive service subject additives was found to be similar, unlike that of the additives sheared by the diesel injector method. These are narrower and more dependent on the initial MWD of the additive, especially in the case of the additives with broader MWD. A shear test that better correlates with service has to take into account not only the control of shear stress, by appropriate calibration, but also a suitable degradation mechanism.