Ionic liquid mixtures with both different cations and anions (i.e., ternary reciprocal mixtures) are often formally treated as binary mixtures. Mixing laws for binary mixtures are inappropriate for ternary reciprocal mixtures as they do not account for both attractive and repulsive interactions between ions in those liquids. In this work, the viscosity of the [C(2)py], [C(4)py] // Cl, Br ternary reciprocal system (where [C(n)py] = 1-alkylpyridinium) and all its common-ion binary and unary subsystems was measured over the entire composition range from temperatures close to the liquidus up to about 200 degrees C. A new viscosity model was proposed to describe the viscosity of ternary reciprocal mixtures more rigorously by accounting for all ion-ion interactions. The robustness of the proposed viscosity model was discussed in comparison with other approaches proposed in the literature. Anomalous discrepancies for the low-temperature viscosity data were observed close to the center of the reciprocal square (consisting of an equimolar mixture of the four pure salts [C(2)py]Cl, [C(2)py]Br, [C(4)py]Cl, and [C(4)py]Br) and could not be accounted for by any of the approaches considered.