Chances are if the engine has sat for some time the valve seals could be dry which will let oil seep into the combustion chamber. If it doesn't clear up after a short run time you may need to replace these. It can be done with the head on the engine, but it can be a PITA.
When I've had this problem in the past (SBC) but they only smoked on start up. The seals let oil in when I let the engine sit overnight. Once the small amount of oil burned off at start up it was good.
