Rocker cover off really bright torch and look through all the valve springs to see which if any on the valve guide seals has lifted off its seat, could also be the thinner oil you are using, well worth checking valve clearances when you are in there as dead common for valve lash caps to break, cost pennies, so common worth keeping a couple on the shelf
200TDI always make some sort of smoke no matter what people say theirs does, dirty old things but tough as old boots so long as oil changed often