What you could do, I suppose, Ive never done it so take it for what its worth. But what you could do is take a couple gallons of Amsoil coolant to a shop (or I suppose you could ask first) and see if they will flush the system using that instead of what they would use and have it professionally flushed. That way they could run water through the system with their machine until all the old coolant is out and then add in the Amsoil coolant at the proper mixture.
I dont know if there is a better sealing cap, I do know that if you go get a new radiator cap from the dealer they are of a different style or design now. If its improved or not I cant say, its just different than the one the car came with from the factory, at least on mine anyway.
