1995 Geo Metro Engine smokes but only on highway

Hi, This question regards why my engine would smoke on the highway but not when driving around town. I replaced the pcv valve and the engine was also rebuilt. I followed the proper breakin procedure. What happens is when I am drving the car under speeds of 50mph it drives fine. But once it goes to higher highway speeds it smokes like crazy. Black smoke to be exact. Oil goes into the air filter area and the more you open the throttle, the more it smokes. But once you get off the highway, it goes away. No smoke, runs great. The car does not smoke when it is cold on startup. When driving on the highway, and it starts to smoke, it loses about a quart within 15-20 miles. I was wondering if this will go away after a while and I shouldn't continue taking this on the highway for 2-300 more miles and see if it goes away or should I just toss this engine and start from scratch.