  We investigate experimentally the influence of the liquid viscosity on the
problem of the generation of waves by a turbulent wind at the surface of a
liquid, extending the results of Paquier, Moisy and Rabaud [Phys. Fluids {\bf
27}, 122103 (2015)] over nearly three decades of viscosity. The surface
deformations are measured with micrometer accuracy using the Free-Surface
Synthetic Schlieren method. We recover the two regimes of surface deformations
previously identified: the wrinkles regime at small wind velocity, resulting
from the viscous imprint on the liquid surface of the turbulent fluctuations in
the boundary layer, and the regular wave regime at large wind velocity. Below
the wave threshold, we find that the characteristic amplitude of the wrinkles
scales as $\nu^{-1/2} u^{* 3/2}$ over nearly the whole range of viscosities,
whereas their size are essentially unchanged. We propose a simple model for
this scaling, which compares well with the data. We finally show that the
critical friction velocity $u^*$ for the onset of regular waves slowly
increases with viscosity as $\nu^{0.2}$. Whereas the transition between
wrinkles and waves is smooth at small viscosity, including for water, it
becomes rather abrupt at large viscosity. Finally, a new regime is found at
$\nu > 100-200 \times 10^{-6}$~m$^2$~s$^{-1}$, characterized by a slow, nearly
periodic emission of large-amplitude isolated fluid bumps.
