I would say it is pretty much normal, if it was oil , you would see it injectors too would be smokey, the exhaust system is full of soot that s why they never rust out on a diesels just fall apart from vibration. soot is normal on all diesels. and yes valve seals would show as blue smoke at crank up from running down the stem and setting on top of the piston over night. thats my 2 cents worth old time diesel mechanic.