The nonlinear shear theology of a hyperbranched polyisobutylene (PIB) with narrow molecular weight distribution (M-w/M-n = 1.3) was compared to that of a polydisperse (M-w/M-n = 2.5) linear PIB. After adjusting the respective measurement temperatures to yield equal shear viscosities, the theology was found to be quite similar for the two polymers, notwithstanding their markedly different structures. These similarities persisted in capillary extrusion experiments at moderate shear rates, on the compounds reinforced with carbon black, However, at higher shear rates, the linear PIB exhibits a greater elastic response, with consequently larger extrudate swell.