Gerald is certainly right about that last 20%, Karl. Even 5-10% is a significant loss of cooling exposure when you need every square inch you can get for those hills. It's common to read in Forums and magazines about coach owners with rear radiators experiencing the oil blow-by problem. Those complaints were one reason, while shopping 5 years ago, that I ignored all coaches on a dealer lot that didn't have side radiators. Primarily I mostly wanted easier engine view and access.
The residue sticks in the fins where it attracts dirt. Even if that combination doesn't plug off inter-fin space entirely, the coating on the fins radically reduces heat exchange. You're going to have to break up the oil residue with some sort of foaming solvent or at least detergent to get it to release from the fins; allow it to sit and work, and then try Tom's idea to brush and flush it and its dirt out of there. As Gerald and Ed say, you don't want to remove the radiator if you can help it. Ed's blowgun extension is worth a shot for large debris, but likely won't unstick that oil.
Another issue with the blow-by that many complained of was that it also coated their toad. Seems like some did more than just rerouting the tube, and managed to keep the crud off their car too with further tube modification. Forgive me, I don't recall what that extra step was, but perhaps others here can proffer input.
Joel
