The role of the viscosity ratio kappa during forced imbibition in porous media is investigated theoretically using a new simulator, and experimentally based on displacements in model pore networks. Both theory and experiment show that kappa is an important parameter of microdisplacement in porous media, not only for intermediate and large capillary number values, but also for small values, say Ca < 10(-6). In the latter region the residual nonwetting saturation S(or) is virtually independent of Ca for kappa < 1, and increases weakly with decreasing Ca for kappa > 1. The unexpected result is that, even for very small values of Ca, S(or) decreases appreciably as kappa decreases, especially in the case of very good wettability. Simulations indicate that the effect of kappa on S(or) for low Ca values is enhanced as the contact angle decreases. The phenomenon is attributed to a synergistic effect between capillary microfingering and localized viscous forces. It must be emphasized that the velocity gradients which are created locally by the advance of a single meniscus, or of a wetting film, are sufficiently large to make viscous stresses important, even when the Ca value of the macroscopic flow is very low (say, of order 10(-8)). A favorable viscosity ratio reduces the extent of capillary microfingering and thus increases the efficiency of microdisplacement, despite the fact that for Ca < approximately 10(-6) the viscous stresses are negligible on a macroscopic scale. Further work is needed to analyze the synergism between kappa and theta(c) at low Ca values.