Get some miles on it, a possible cause is the oil which accumulates in the sump on standing being scavenged, hence exhaust clears after a short distance. Try leaning the bike to the left on start up, left cylinder may also start to smoke. No noise or other symptoms? Oil returning as it should in a stream and gulp on idle? All OK? Ride like you stole it to break the bore glaze, yes, a DAMN GOOD IGNORING.
Modern oils, our fastidious cleanliness and pussyfoot break in procedures often result in the scenario outlined by Slymo. I go for the cheapest oil, the "suitable for older engines plain wrapper Supermarket Own Brand cheapo crap" for the first fill. The lubricating properties are adequate, but at this price it does not contain the exotic ant-wear, anti oxidant and other additives that contribute to this initial problem of poor ring sealing.
Swarfy.