most likely cause is that the valve seals have not reached full operating temp so a little oil gets by after shut down.

 

I had this exact same problem with my 2001 f150 4.6 engine I had 14000 miles on the engine , The dealer did a compession and leak test on the engine and found that the no. 2 cyinder failed the leak test, it was determined that the number 2 cylinder did not meet tolerences , Replaced the engine [ FQR]