If the engine is left overnight any residual oil on the valve stem area runs down past the seals through the guides and down on to the valve seat where it can pool when the valve is shut. Granted there will be some valves open and this will cause the oil to run straight into the cylinder.

With the engine is running it is unlikely that the oil will travel past the seals etc quick enough to really notice it smoking unless of course the seals/guides are totally shagged plus the fact that due to thermal expansion of the valve/guide this will reduce the gap anyway..

When the engine has been left overnight the oil that has collected at the valve seats is drawn into the cylinders and burnt causing an initial smokey condition.

CheeRS, Mike.