Classical linear stability theory is extended to include the effects of temperature dependent viscosity and density. From an asymptotic point of view, i.e. after a Taylor series expansion of all properties with respect to temperature and pressure, they turn out to be the leading order Variable property effects for forced convection at low speeds. In an asymptotic approach assuming small heat transfer rates, the two property effects acting on the basic flow and its perturbations are well separated from each other. The asymptotic solutions hold for all Newtonian fluids. Examples are given for air and water. The numerical results are in good agreement with experimental data from the heat transfer literature.