You didn't mention if the engine is using a PCV valve or it has a road draft tube coming from the rear hole on top of the valve cover. One possibility is if you are using the older type of vent the has the road draft tube then it could be coming from it at higher rpm if there is no baffle in the valve cover or the baffle was removed. A sure sign of this being the problem would be the top, sides, and bottom of the engine completely oil free and the lower end of the road tube dripping wet. This would soak the underside of the car as you describe and leave the engine relatively dry forward of the tube.
