The basic answer is that the relationship between torque or power is not linear with respect to viscosity, except in laminar conditions (impeller Reynolds number less than 1.0).  These linked files show how to calculate the dimensionless impeller Reynolds number (note Reynolds number is dimensionless so the units used for the variables must cancel each other).  The Power Correlation  file shows the factor that should be applied for the effect of viscosity, which applies to torque when volume, agitator, rpm, temperature and geometry are the same).  If there is "no turbulence," Reynolds number less than about 5.0, doubling the viscosity will double the torque.

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