This is not unusual, the crud left behind by the original dino oil is being shifted by the synthetic oil, resulting in imperfect seals. Assuming you are using the appropriate grade of synthetic oil, you have a couple of options:-
1) Check the torque on the oil pan bolts and keep a close eye on the oil level for the next 5k in the hope that as the synthetic oil works its magic, the seals have not sustained terminal damage and the leak stops; or
2) Start replacing seals and run synthetic oil from day 1.
I have successfully converted from dino to synthetic on an older BMW, but initially changed the oil pan gasket and ran semi-synthetic oil for 2 changes before converting to the fully synthetic. If the car has been serviced regularly and was using a good dino oil, the pan gasket may do it for you. Degrease the engine thoroughly and use a torch to check the source of the leak after driving the car for a short distance.