Plymouths and other Mopars have always had hardened valve seats. This is one area that wears without the protection of lead. The only additive I use in my old Plymouths is Zinc and even that may not be needed but sinces it is realitively cheap, I use it as insurance. I belong to a Plymouth Owners Club and no one I know adds Lead Substitute to their cars. And no one has lost any valve performace either.