The most important mixing rules reported in the literature to calculate the viscosity of blends of crude oils were tested. Experimental kinematic viscosity of 47 binary and tertiary blends of crude oils were used to determine the accuracy of the mixing rules. Conventional mixing rules showed poor prediction capability, while those based on interaction parameters presented low errors. The mixing rule with 4 interaction parameters resulted to be the best approach, which was confirmed by a series of statistic techniques and sensitivity analysis of the determined parameters.