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What are you using for oil- weight and brand?
Did they check to see if any oil was in the coolant?
If you are burning oil in small enough amounts the cat converter will filter out the smoke.
You can trying changing brands of oil (although a little difficult if you don't know what you've used in the past). If you are confident this condition exists without regard to the brand of oil, the problem is likely the valve seals. They gave some trouble.