We study the motion of a viscous slug in a capillary tube, where one of its interfaces is in complete wetting while the other is in partial wetting conditions. When the partially wetting interface experiences strong pinning, the motion of the slug crosses over from stick-slip to steady sliding as its speed increases, reminiscing the dynamics of frictional sliding of solids. We characterize this transition using a combination of experiments, analytical arguments, and a computational model. Notably, this system can operate in either constant-rate or constant-force settings, which, in the limit of low displacement rates, allows probing the respective force-to-speed scaling relations by Raphael and de Gennes ( J. Chem. Phys. 1989) and Joanny and Robbins ( J. Chem. Phys. 1990).