$\alpha$, the shock oscillation is irrevocably unstable. We change the injection parameters to $v_{\rm inj} = - 0.06532$, $a_{\rm inj} = 0.06221$, and ${\lambda}_{\rm inj} = 1.7$ at $r_{\rm inj} = 50.8$, and plot the time evolution of the shock $r_s$ for inviscid flow (Fig. 2d), $\alpha=0.003$ (Fig. 2e) and $\alpha=0.007$ (Fig. 2f). The mechanism of shock oscillation due to the presence of viscosity, may be understood in the following manner. We know viscosity transports angular momentum ($\lambda$) outwards. Since the post-shock disc is hot, the angular momentum transport in the post-shock disc is more efficient than the pre-shock disc. Accordingly, angular momentum piles up in the post shock region. On the other hand, a steady shock forms if the momentum flux, energy flux and mass flux are conserved across the shock. Therefore, as the angular momentum piles up in the immediate post-shock region, extra centrifugal force will try to push the shock front outward. If this piling up of $\lambda$ is moderate then the expanded shock front will find equilibrium at some position to form steady shock (Figs. 2b, 2e). If this outward push is strong enough then the expanding shock front will overshoot a possible equilibrium position and 