My 1997 4.6 does that too. Normally it only happens after I start it cold, move it somewhere different in the drive way (doesn't get to warm up is the point) and shut it off. Then the next morning when I come to start it it puts out a nice puff of blue smoke. My experience would say that it is a valve seal issue. They shrink and let a little oil drip into the chamber when you start it cold. So that built up oil on top of the piston burns off when you first start it. Not really anything to worry about, but you ALWAYS want to take it easy on your engine until it's warmed up. No worries though, unless you really want to take your heads apart and replace the valve seals.