In the Stout book by Lewis, he mentions that oats produced a harsher flavor. With respect to Jim's quest to restore body in that attenuated wort, I suggest that flaked wheat is a very good option. I find its flavor relatively crisp and clean. A percent or so will definitely add strong head building to the beer. You could also use flaked barley, but I found that it does have a distinctive flavor that doesn't meld well with pale worts.