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.
Thanks everybody! Thank you Dr. Chafin; I am planning to use Chevron's equation for calculating the viscosity of a mixed oil. https://neutrium.net/fluid_flow/estimating-the-viscosity-of-mixtures/