Atomistic molecular dynamics simulations were carried out at equilibrium to calculate the shear viscosity of n-decane and n-hexadecane under ambient and high temperature-high pressure conditions. Two force fields, one using a computationally efficient united-atom (TrAPPE-UA) approach and another an all-atom (AA) approach (Tobias, Tu and Klein), were employed. Quantitative agreement with experimental data is obtained with the AA force field, whereas the UA model predicts the viscosity within 20-30% of the experiment. The intra- and inter-molecular structure of the fluid obtained from these two models is similar, pointing to the role of differences in their dynamical characteristics to the observed difference in the calculated viscosities.