It is most likely from running rich while idling in hot traffic. One way to check if this is the case is to simply pull your plugs and check the color (see if they're red or black... if they're black... then yes, you're running rich... meaning there's a lot of unburned fuel left in the cylinder after each ignition cycle... and that fuel washes into the oil). You could adjust the heat range of the plugs you're using, but since the car is so new, it's best to defer to the dealership. You're paying for a warranty, might as well use it. ;)