The ability of oil to retain its viscometric properties is particularly important in Heavy Duty Engine Oil applications to prevent wear and maintain intended levels of oil pressure. It is known that mechanical shearing of the oil, fuel dilution, oil oxidation and soot level all affect the aged oil kinematic viscosity at 100°C (KV100). For API CJ-4, as well as for many OEMs, an oil's KV100 must stay within the original viscosity grade as defined by SAE J300 after 90 cycles in the Kurt-Orbahn (KO) apparatus. This study investigates the effect of polymer chemistry and structure on extended shear stability of lubricating oils by evaluating the performance of two Viscosity Index Improver (VII) chemistries, Olefin Copolymer (OCP) and Hydrogenated Styrene Isoprene (HSI), under more severe shearing conditions than required for CJ-4. These technologies were evaluated in the KO shear test up to 700 cycles and the KRL shear test up to 8 hours. The KRL has been proposed as a possible alternative to the KO test, and extended KO shearing beyond 90 cycles provides a better indicator than the standard 90 cycle test for performance under severe conditions, as demonstrated by historical field test data. The rate of viscosity loss for OCPs diminishes to a low plateau after 90 cycles in the KO test, while viscosity loss with HSI chemistry continues at a high steady rate well beyond 90 cycles. The difference between chemistries is attributed to polymer structure and molecular weight. In conclusion, OCP chemistry is found to provide advantages in retaining viscosity after extended shear.