  Viscous solitons are strongly non-linear surface deformations generated by
blowing wind over a liquid beyond a critical viscosity. Their shape and
dynamics result from a balance between wind drag, surface tension and viscous
dissipation in the liquid. We investigate here the influence of the liquid
viscosity in their generation and propagation. Experiments are carried out
using silicon oils, covering a wide range of kinematic viscosities $\nu_\ell$
between 20 and 5000~mm$^2$~s$^{-1}$. \modif{We show that, for $\nu_\ell>
200$~mm$^2$~s$^{-1}$, viscous solitons are sub-critically generated from an
unstable initial wave train at small fetch, where the wind shear stress is
larger. The properties of this initial wave train are those expected from
Miles's theory of the Kelvin-Helmholtz instability of a highly viscous fluid
sheared by a turbulent wind: the critical friction velocity and critical
wavelength are independent of $\nu_\ell$, and the phase velocity decreases as
$\nu_\ell^{-1}$.} We demonstrate the subcritical nature of the transition to
viscous solitons by triggering them using a wavemaker for a wind velocity below
the natural threshold. Finally, we analyze the flow field induced by a viscous
soliton, and show that it is well described by a two-dimensional Stokeslet
singularity in the far field. The resulting viscous drag implies a propagation
velocity with a logarithmic correction in liquid depth, in good agreement with
our measurements.
