2016 XF Prestige; The gloss black exterior B-pillar trim has turned dull. I've tried Plexus plastic polish to little effect. Have also tried Vaseline and baby oil; temporary, of course. Any suggestions? Many thnx...

 

I had the same problem on my old 2010 XFR.
I tried various things to no avail but what eventually worked for me was Meguiar's Ultimate Compound polish and a microfibre cloth.
Not quick or easy, took several applications and lots of hard rubbing, but eventually achieved an as new mirror finish.