Jenks,
My guess would be that its cosmoline! And from the few Mark X actions that I have seen to include a Parker Hale .30-06 there apparently was no shortage of it in Yugoslavia where they were built. The last Mark X I purchased was just the action that I later went on to build a custom rifle; it was thick with cosmoline. It took me using gasoline and a brass brush to clean it out. I suppose over the course of time it could stain the metal. I love those actions. Heavy but dependable.