Abstract: A rotary seal is provided that operates hydrodynamically in response to relative rotation even when subjected to exposure to a high fluid pressure from either side, and provides low breakout and running torque. A dynamic sealing lip having a wavy geometry provides hydrodynamic lubrication in response to relative rotation when little or no differential pressure is acting across the seal. When differential pressure exists, controlled fluid pressure-induced distortion results in a distorted dynamic sealing lip configuration that is suitable for hydrodynamic lubrication, regardless of which direction the differential pressure acts from.