I doubt that the factory finish is Tru-oil, so that would be the first problem. If you spot finish it will most likely show where you did it. Yes you can stain walnut with walnut stain (it's just the color name) Walnut tends to have light and dark contrast to it sometimes, some like it that way, I do. If you want the whole thing darker, you will have to strip the entire stock and stain it to your liking then apply the Tru-oil.