What makes oolong tea “slippery”?

I’ve noticed that oolong tea (Dan cong and rock tea are what I’ve tried) has a particularly “slippery” mouthfeel that I haven’t noticed so much in other teas. What causes this, and are there other teas that are known for it?

(Honey orchid photo for tax - this is my favorite dan cong and what I happen to be drinking this morning.)