I had this problem with a van several years ago. The problem started overnight because a valve seal cracked, probably due to age, and allows oil to seep down the valve stem. While the engine is running, you won't see the oil as it burns off because of the small amount but there is an accumulation of oil ends up in the intake manifold after the engine sits for a period of time. This is why the visible smoke is only seen upon start up.
Taking the valve cover off probably won't help because it is hard to see the valve seals well enough to tell if they are ALL still sealing. To be sure, you must dis-assemble the valve and valve springs, and take all the seals out and inspect each one. It only takes one to be your problem too.
If you determine this to be your problem, I would recommend a referbished head as a replacement as opposed to having your rebuilt. It will probably be cheaper. Bruce
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