If it's the stems and they appear to be turning olive drab, this is not oxidation,...I know that's what they call it, but it's not. When rubber is vulcanized, sulfur is used in the process....over time the sulfur begins to be released due to a weak chemical bond. The yellow color is that sulfur.
Sodium hypochlorite, (good old Clorox bleach), turns sulfur black. It appears to remove it but it doesn't...it just makes it harder to see.
To do this I use a tall glass and fill it with Clorox taller than the stems that will soak in it for one hour.
This process will rough up the surface of the stem which will require light sanding and buffing to bring back to show-room new.
Before doing this one hour bleach soak…. I fill the end holes with Vaseline to keep the bleach out of the draw-hole and also apply a little Vaseline to any emblems you want to save,….they will be destroyed if you don’t. Aluminum emblems like GBD pipes will also be destroyed.