Dear Andrew,

I would expect the viscosity of completely miscible liquids to show a linear relationship in terms of mixture composition. Some liquids that are miscible on a macroscopic scale, however,  such as Ethanol and Water show synergistic effects resulting in a non-linear profile. See for example this application note on Ethanol-Water mixtures and cited references.

http://www.malvern.com/en/support/resource-center/application-notes/AN140821-Evaluating-Viscosity-and-Viscous-Synergy.aspx

Best Regards

John

Check out the Refutas equation (2000).  It is phenomenological but is analogous the the eutectic equations for specific heats of alloys.   

You may refer to: B.E. Poling , J.M. Prausnitz, J.P. O'Connell, "The Properties of Gases and Liquids", 5th ed., McGraw-Hill, 2001 (Section 9-13).