Multigrade automotive lubricants contain polymeric viscosity modifiers which enable the oil to provide adequate hydrodynamic lubrication at high temperatures and good starting/pumping performance at low temperatures. Under operating conditions in engines, transmissions and gear boxes, polymeric additives undergo both temporary and permanent viscosity loss. The former is caused by flow orientation and the latter by molecular chain scission. Whatever the mechanism, original equipment manufacturers are interested in maintaining a minimum level of hydrodynamic viscosity from oil change to oil change. This is often expressed as a “stay-in-grade” requirement.Commercial viscosity modifiers (VM) span a wide range of chemistries and molecular architectures. An earlier paper (1)1 reported on the permanent viscosity loss characteristics of SAE 5W-30 engine oils differing in VM chemistry and speculated that differences in molecular structure - linear chains, A-B block copolymers and stars - could explain most experimental observations. This paper presents a similar study on a series of SAE 15W-40 heavy duty diesel engine oils formulated with OCP, styrene/butadiene, star and PMA viscosity modifiers. Very similar relative permanent viscosity losses to those reported earlier were observed. To provide direct experimental evidence for molecular chain scission, a third set of experiments were carried out in which binary polymer/oil solutions were subjected to mechanical shear degradation according to ASTM D3945. The molecular weight distributions of the new and sheared oils were measured by gel permeation chromatography, and the results are shown to support the molecular degradation mechanisms proposed in the first paper.