Pulled my ex manifolds to fix flange leak and noticed a white residue in one port and saw a really tan colored valve. All the others look the same, pretty normal for a mega-mile engine. its a 318
Probably an additive of sorts in the fuel. As to why only one valve.....perhaps that hole is up/down on compression.....weaker/stronger spark......looser valve guide.......just put the manifold back on and it will go away.....because you will not be able to see it anymore...