The viscosity for acoustic fluid elements 30/220/221 is a bulk viscosity term, not a shear viscosity term. Set up a modal (FLUID30) and harmonic analysis (FLUID80). The excitation for the FLUID80 model could be base excitation for the cantilever beam (for example). Then, check the results (e.g., response at tip) to see if all of the modes of interest can be captured. Once this is done, one can add damping (MP,VISC) in the FLUID80 model to see how the frequencies change with viscosity.