I usually stay clear of discussions regarding correct colors, and weathering because they can be so subjective and can get out of hand very quickly. So, at the risk of stirring the pot, and with all due respect and admiration for one of our hobby's most prolific developers, I will comment on this particular subject.
First off, I agree with Jan re the current C-47/DC-3 fliers. For the most part they are kept in pristine condition, including the engine cowls and nacelles. However there are many examples of operational military C-47s, and even a few civilian airliners with varying degrees of oil and soot on the nacelles behind the cowl flaps, a few of which are posted below.
So, from this repainter's point of view, I respectfully disagree with Jan's statement in his first paragraph. While I also agree with Jan re that degree of such "weathering" may come into question. But, that is usually determined by many factors such as personal taste, skill level and experience with repainting. As for myself, I repaint subjects for me. Most times I try to make them as accurate as possible. However, there are times when I take artistic license with such things, and because it's for me, it's OK if others don't care for them.
Finally, this is not a debate or argument. It is my humble opinion only, and I will not engage in any back and forth because I'm not trying to convince anyone of anything. Just sharing an opinion.
Steve