You're getting foam like that in the middle because the middle section being the hottest part of the pan. Density might be a tiny bit higher there because of that. Can't see your arch, but I suspect it is a good bit cooler on the sides. Is your arch insulated?

In any case, a bit of spilling over isn't going to hurt, especially if you're not trying to form a gradient. I wouldn't use any oil unless it starts really foaming over in the side channels as well.