If the Dunhill is newer, say 1950 or later, there’s not much to worry about protecting the dot. It’s a form of plastic. Older ones may have a cellulose dot, which I’m guessing might be damaged by bleaching. The safest thing here is to not use Oxyclean. I use it myself sometimes, but only when the oxidation is very severe, and the oxidation on your stem is fairly light. I’d just use the Magic Eraser sponge to start, then follow up with some automotive plastic polish or a silver polish. When it’s looking good, add some mineral oil or Obsidian oil and you should be happy. You could also send the stem to a repairman like Norwoods or Tim West and they can buff out the oxidation.