Yeah I have a set of M/T finned valve covers for a small block Ford and they are all stained like that in the fins from oil. I still have them but changed them. I will probably powder coat them and pc the inside of the fins black so it's not noticable as much.
All ball milled stuff would look really nice. Too bad I can't afford it. Billet Specialties stuff is really expensive but their valve covers, air filters, are breathers, etc in the ball milled look nice.