Two things help with your problem....make your wicks more solid and put the fill screw in. Just make sure to make your wicks smaller diameter than the wick hole or you will have serious wicking issues with the fill screw in. The juice getting darker and darker does't have any thing to do with an oxidized mesh. Oxidized meshes do however give off small carbon particles that start to float in your juice....that is a good reason to never oxidize your meshes.
I've been having the same issue. More mesh in the wick makes sense, I'll try that, but why is the fill screw needed?