Wood you see scratches on typically has a surface finish by an oil or something similar. It makes the surface darker than natural wood. The scratch will expose natural wood that is lighter in color.
Walnuts contain as you say oil and when it get into the wood in the scratch it get darker and is a lot harder to see. How well it works will depend on the color of the wood and what is on it, it might not get dark enough and something else needs to be used.
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