My 2001 W203 C320 was belching out bluish smoke on startup after sitting off for a while, or after idling for a long time (like on a congested motorway). Other than that, no smoke during normal driving. It was also consuming a fair bit of oil. Definitely not head gasket.

Took it to an indy and he said it was valve stem seals, he ruled out piston rings since the smoking didn't happen all the time and even taking it to 4000rpm up a hill didn't necessarily produce it. He also replaced valve cover gaskets and spark plugs at the same time. PCV system was ok.

I took the car back and went for a 40 minute city drive, to my horror I saw smoke billowing behind me when I took off after spending ages idling in a congested motorway, cleared up after a few seconds. I went home and left it sitting in the garage for a few hours and started it again, I could see copious amounts of smoke coming from the exhaust, just like the original problem. My country's got a 3 day public holiday and I won't be able to contact any mechanic till monday. I'm at a loss to understand, if it isn't rings or seals, how else can oil get burnt? I did a bit of googling and it could be valve guides but shouldn't they have found that if they were replacing the stem seals?