In the petroleum industry, the viscosity of mixtures containing thousands and millions of chemical species must be predicted over large temperature ranges. Traditional models are empirical based on bulk properties. If such models are applied to mixtures other than those for which they were developed, large errors may occur. New methods proposed calculate the viscosity of petroleum fractions over large temperature ranges, using a new mixing rule reflecting observed positive and negative mixing effects. The temperature range of equations for calculating pure component viscosity is extended to the entire liquid-phase range. If solid components are dissolved in the liquid, the viscosity increases sharply with further temperature reduction. This effect is captured in the new model by incorporating the freeze point distribution. For converting between absolute and kinematic viscosity, a new equation is proposed for the temperature effect on density.