  Direct numerical simulations are performed of turbulent forced convection in
a channel flow with spanwise wall oscillation, either as a plane oscillation or
as a streamwise travelling wave. The friction Reynolds number is fixed at
$Re_{\tau_0} = 590$, but the Prandtl number $Pr$ is varied from 0.71 to 20. For
$Pr>1$, the heat transfer is reduced by more than the drag, 40% compared to 30%
at $Pr=7.5$. This Reynolds analogy breaking is related to the different
responses of the velocity and thermal fields to the Stokes layer. It is shown
that the Stokes layer near the wall attenuates the large-scale energy of the
turbulent heat-flux and the turbulent shear-stress, but amplifies their
small-scale energy. At higher Prandtl numbers, the thinning of the conductive
sublayer means that the energetic scales of the turbulent heat-flux move closer
to the wall, where they are exposed to a stronger Stokes layer production,
increasing the contribution of the small-scale energy amplification. A
predictive model is derived for the Reynolds and Prandtl number dependence of
the heat-transfer reduction based on the scaling of the thermal statistics. The
model agrees well with the computations for Prandtl numbers up to 20.
