Answer by Lisa Lin:
They both exist:
– Natural milk oolong is called jinxuan oolong, and is from Alishan Mountains in Taiwan. However, most people find the original milk taste not strong enough.
– Other oolong teas with artificially added milk flavor can be made anywhere in Taiwan or China.
By the way, the fact that the milk flavor is artificially added, doesn't mean it's a low quality tea. I have seen many flavored milk oolongs before with really high quality leaves.