Arrhenius equation
\ln {\mu}_{12} = x_1 \cdot \ln {\mu}_1 + x_2 \cdot \ln {\mu}_2
Lederer-Roegiers equation
\ln {\mu}_{12} = \frac{x_1}{x_1 + \alpha \, x_2} \cdot \ln {\mu}_1 + \frac{\alpha \, x_2}{x_1 + \alpha \,x_2} \cdot \ln {\mu}_2
Grunberg-Nissan equation
\ln {\mu}_{12} = x_1 \cdot \ln {\mu}_1 + x_2 \cdot \ln {\mu}_2 + \epsilon \, x_1 \, x_2
Oswal-Desai equation
\ln {\mu}_{12} = x_1 \cdot \ln {\mu}_1 + x_2 \cdot \ln {\mu}_2 + \epsilon \, x_1 \, x_2 + K_1 \, x_1 \, x_2 \, (x_1 - x_2) + K_2 \, x_1 \, x_2 \, (x_1 - x_2)^2
The Lederer-Roegiers equation is reported to be the most accurate among single-variable models.
Physics / Fluid Dynamics / Mixing Rules
Boris Zhmud, Viscosity Blending Equations, Lube-tech, 121, 2014