The models studied in this work are the classical Newtonian and Oldroyd-B models, as well as their generalized (shear-thinning) modifications. Numerical tests are performed on three-dimensional geometries, namely an idealized axisymmetric stenosis and a realistic stenosed carotid bifurcation reconstructed from medical images. The numerical solution of the system of governing equations is obtained by a finite-volume method on a structured grid. Model sensitivity tests are achieved with respect to the characteristic flow rate to evaluate its impact on the observed non-Newtonian effects.