Toyota Repair: 2004 sienna smoke on startup, valve stem seals, toyota dealer

Question
My question
Hi 2004 sienna oil problem but not the regular stuff. 
Burns oil in a stinky cloud but only at startup for minute or so at worst. Then if engine warm nothing.  About half the time you start it up cold and nothing happens. It's as if some intermittent condition when I stop allows oil to trickle past something and collect then burn off on start. But situation somehow does not occur while driving as the van is really not burning oil in general. Running Amsoil full synthetic.   Thx for any clues

Toyota dealer said it was needing whole new engine but I don't buy it as that was three years ago and other than the symptoms no issues. But we are getting tired of this. Also have aftermarket belt skweeking. But I think I'll try Toyota belt as this is not the big problem. 

Mikes historical answer

What happens is the oil gells up in the valve covers and lays there. The pcv sucks it up before it can drain back down to the oil pan and whala, smoke.Only takes a small amount to smoke.

This answer seems perfect but I h ave few questions. What is pcv  normal function? If oil is thick how is it sucked up  ? Having trouble picturing this i guess  And if I go to effort of cleaning the valve cover area how long will this stay clean?  I run full synthetic so I expect it to put up with more heat than most oils.

Answer
If there is any oil gelling going on at all it usually affects the valve stem seal, they will start to get brittle from the heat which also causes the oil to gel so the solution would be to replace the valve stem seals which can be done without removing the cylinder heads, most of oil deposits can be cleaned while the valve covers are off.