We studied the validity of using the Navier-Stokes approximation for the viscous stress tensor and heat flow vector in global thermospheric flow situations. Our aproach was based on the completed 13-moment system of transport equations, which is applicable to both collision-dominated and collisionless flows. We found that the classical Navier-Stokes expression for the heat flow is valid at all altitudes, i.e., the heat flow is proportional to the neutral temperature gradient. However, the Navier-Stokes expression for the stress tensor is not valid above about 400 km for the off-diagonal elements and above 150 km for the diagonal elements due to the direct effect of heat flow on stress. Nevertheless, the corrections to the Navier-Stokes stress tensor expression have less than a 10% effect on the wind speed for a wide range of thermospheric conditions.