Wall slip has been pointed out as a way to reduce cracks and swelling on polymer extrudates. To investigate this phenomenon, the capillary extrusion of a purely viscous generalized Newtonian fluid is computed with the finite element code POLYFLOW with a realistic slip law. This fits with the friction curve data for a polydimethylsiloxane (PDMS) in a steel die. It is based on molecular dynamics theory and contains a critical stress below which there is qualitative adhesion at the wall. The existence of a slip boundary condition is shown to modify the morphology of the velocity field which tends toward a plug flow. It thus largely reduces the fully developed stress level and the exit stress concentration. Localized slip at the die exit appears even for fully developed flow under shear rate values which are lower than the critical sheer rate corresponding to the occurrence of slip at the wall in a Poiseuille flow. This is explained by exit stress concentration. The effect of slip length is then examined: it is found that exit stress and extrudate swell can be largely reduced by using a very short slip length only. This result shows the interest of optimising slippery surfaces in the design of extrusion processes.