| As the engine settles down from driving, the oil that has puddled in the cylinder head will drain back down into the oil pan. but some will remain on top of the valves and springs. There are umbrella type seals that fit snug up against the valve stems sticking up through the head. These seals will get very hard over time and start to degrade.
Oil seeps by them and ends up wetting the combustion chambers. When the next start up occurs, the oil is quickly burned and a big puff of blue smoke will exit the tail pipe. | 
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