Julia Child said, don't crowd your mushrooms if you want them to brown nicely (ok, I learned this from the movie Julie & Julia, so what?) - but I disagree!
So do these folks, in fact. In practice, it always seemed to me that mushrooms end up leaking tons of moisture as they cook and virtually de-glaze themselves from the pan, negating the need for any cooking oil. And if you did use oil, the mushrooms would just absorb it too quickly.