I saw this on a you tube video by Richard Michael Owen. My oil cap was looking a little ratty so I brought it to work and put it on a lathe and held sandpaper up to it as it was spinning. Started off with 400 grit and worked on up to 3000 grit and finished with some mothers mag aluminum polish. I think it looks great. Heres a before and after photo.

 

You can polish anything aluminum with a simple buffer. There are plenty of good aluminum polishes out there but one trick Ive found is once you have things polished rather than spending a lot of time trying to get the black residue off just use flour. Yes regular old cooking flour.
Dust it on and wipe it off. Really helps the shine.