Might try this...
Here is what I posted last August, only got one response suggesting WD40.
"I ran into a guy who takes pride in the appearance of his ride who says he uses Linseed Oil (70%) with paint thinner (30%) to restore sun faded black running boards, side mirrors, and other exterior plastic trim. Sure looked good and not at all slippery, a real consideration on the running boards.
Wondering if anyone here has tried this concoction. As I understand, it is important to not exceed 30% thinner. And, no, I didn't ask which specific thinner he used."