Just bought an 88 635 with 145000 miles. I was told that the rear main was leaking. When I put it on the lift, I can see the oil coming from up high somewhere around the back of the head. I know it's not the valve cover, because I can see that much of it, and I pulled the valve cover off and the gasket looks brand new. Any ideas? I don't want to do a lot of work that I don't need to.