  We experimentally investigate shear thickening in dense granular suspensions
under oscillatory shear. Directly imaging the suspension-air interface, we
observe dilation beyond a critical strain $\gamma_c$ and the end of shear
thickening as the maximum confining stress is reached and the contact line
moves. Analyzing the shear profile, we extract the viscosity contributions due
to hydrodynamics $\eta_\mu$, dilation $\eta_c$ and sedimentation $\eta_g$.
While $\eta_g$ governs the shear thinning regime, $\eta_\mu$ and $\eta_c$
together determine the shear thickening behavior. As the suspending liquid's
viscosity varies from 10 to 1000 cst, $\eta_\mu$ is found to compete with
$\eta_c$ and soften the discontinuous nature of shear thickening.
