Cratering and plastic deformation induced by individual MeV ions on the surface of polystyrene thin films of different molecular weights (M_w) (from 3250 to 2×10⁷u) are investigated using scanning force microscopy. 20 MeV, 85 MeV and 197 MeV gold ions are used to bombard the targets at grazing incidence (79° to the surface normal). Induced surface tracks consist of an elliptical crater followed by a hillock elongated in the direction of the ion incidence. For a given ion energy, the crater size is largest on the lowest M_w film. Crater dimensions are systematically reduced on films of heavier macro-molecules, up to a molecular weight of about 1.6×10⁵u. For M_w > 1.6×10⁵u, the crater size remains approximately constant. The difference observed for the lateral dimensions of the craters are about 50% when comparing the lowest and the highest M_w films at a fixed energy. The observed saturation of the crater size for high M_w values coincides with the onset of entanglement effects in the polymer, which influences the viscosity and the compliance of the material. Moreover, the curve of the crater size versus M_w follows the same trend as the reciprocal viscosity (η^-1) versus M_w, indicating that the viscosity is governing the final lateral dimensions of the craters. The hillock dimensions present a weak dependence on M_w, above a threshold at 3250u. The different behavior observed for craters and hillocks is discussed based on the viscoelastic properties of the polymer at different M_w and on the transient heating occurring close to the ion impact site.