The Viscosity attributes can be found in the bifrostLiquidProperties attributes.
The dynamic or absolute viscosity. The strength of the resulting velocity diffusion depends on the ratio of this value to the density (that is, it depends on the kinematic viscosity). If the scene was modeled to the default Bifröst scale of 1 grid unit = 1 m when Maya's Linear working unit is left at the default value of 1 cm, and density is set in kg/m3, then the value of dynamic viscosity is in Pa·s or equivalently kg/(m·s) (sometimes known as Poiseuille or PI).
With these units, the viscosity of water is 0.001. However, low values have very little visible effect except at very small scales and slow speeds, so in most cases for water and similar low-viscosity liquids you should set this value to 0 to avoid unnecessary computation.
Smooths and dampens liquid flow by blending the simulated velocities with the neighborhood average. This is not physically correct but can be used to mimic viscosity at very little computational cost compared to using the Viscosity attribute. Values over 2.0 may produce unpredictable results over the duration of your simulation.