One of the motivations for EOR methods is the possible instability of the front between phases with high contrast of mobility. Highly viscous polymer slug partially solves the problem by stabilizing the front between water and oil. During further water displacement viscous fingers might appear on the rear end of the slug, and their breakthrough might reduce the oil recovery factor. In the paper we study the size of the mixing zone on the rear end of the slug and further the development of the graded viscosity banks technology (GVB or tapering) to reduce the volume of used polymer without loss of effectiveness.