Bill Tindall, E.Tn.
There certainly is no effect from oil deeper than where light rays reflect. It is not widely known, but well established, that oil followed by varnish is no different in appearance than just varnish. I first learned of this fact in a Bob Flexner article. I suppose the effect of oil, or varnish formulated from oils, has to do with refractive index of the components but that field is beyond my pay grade to explain.
Your questions seem to stem from some misinformation gained from reading something where the author didn't know what they were talking about. A common occurrence in woodworking "science".