Ok, I tried the peanut butter - it actually works! I've only done a couple of the ridges (I had to try it, didn't I?) but I was impressed!! You don't need to put too much on and the brightness comes up when you polish it off.
My cousin's wife has given me some walnut oil to see if that works as well - or is it something specifically in peanut oil?
Well, I went over the black panels with the walnut oil and they've come up nice and shiny. Just hope I don't find there's something in walnuts that doesn't agree with plastic!!
Someone had obviously gone over them before with car polish or something because there was a lot of white in the grooves.
I have also tried the peanut butter on the black panel and it has worked to provide a brightness without leaving any bits on after polishing. I am going to try it on the sides of a tyre to see if that also works. Got the butter in a jar from Asda.
Aha, I got round that by buying two jars - one for the van, one for me! In the end, I used the walnut oil anyway so I've now got nearly 2 jars of peanut butter all to myself.
We always use petroleum jelly on all the black plastic, smear it on thinly and wipe it off, I dont know if it reacts with the plastic or not but we have never had any problems, and it is dirt cheap, does a brilliant job of our bumpers!