Vegetable oil..... Like Canola oil....
An old car enthusiast trick to rejuvenate unpainted, black plastic bumpers that had greyed out from age and exposure. Just put a little bit on a rag and rub it in.
Actually last a much longer time than other treatments like silicone based products and supposedly not harmful to platics. Now whether it reverses in any way any degradation of the plastic, I don't know.
Recently tried it out on a Delrin cupped Stronglight B10 headset that had greyed out and it worked great!