I had a persistant leak from my 351W. Small, but persistant. Very frustrating. I had the car at a shop for some tuning. He had a little machine that injects smoke into the engine. Where the smoke comes out is the same place
oil comes out. Took all of 10 minutes to set up the machine and discover a small leak at the fuel pump block off plate.
Find a shop that has one of these machines and pay the few dollars for the test. Saved a lot of frustration and trial and error leak "fixing".