I'm no pro but on a 318 I used to have I used to get a similar thing. On start up I would get some oil burning then it would go away and there would be no sign of it as I was driving, even under heavy acceleration. If I parked it and let it sit then go back to start it I would get the puff of smoke again at start up. I replaced the valve seals and that helped a lot. (Since the 80 weight gear oil didn't slow oil consumption.
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I did the seal job about 18 years ago so its vague but I did not remove the heads. I just removed the valve cover, plugs, and rocker shafts, both valves on that particular cylinder should be closed after removing the rocker shaft, used an adapter to inject air into the cylinder through the spark plug hole to pressurize the cylinder to keep the valves up, get a valve spring compressor to compress the spring, take out the locks (Don't lose these! Use a magnet!), then remove the seal. Reinstall new seal then everything else in reverse order being careful torquing down the rocker shafts.
If the valve drops in the cylinder you're in deep doo doo , though. At that point you can think of the valve job when you pull the heads to retrieve your dropped valve!