If you find an oxidized contact, NEVER scratch the oxide layer in order to clean the contacts surface, or else it will ruin the contact.

Most of the contacts are covered with various thin metallic layers, in order to minimize the contact resistance and to protect against rapid oxidation. Any fine abrasive paper will scratch the metallic cover layers too.

The above picture is an example of two BNCs from the same cable. This cable is very old, and it was not used in the last 10-20 years. Look at the middle pin of the second connector. It's black because of oxides. You might say to get a new cable, but this is an expensive Tektronix cable, of about 50 USD.

Still, the middle pin of the first connector is shiny. That is because it was cleaned using a pencil eraser. Pencil eraser is extremely good at removing oxides without scratching. Also, pencil eraser is elastic, but solid enough, and can be bent and cut to follow the surface to be cleaned.

The following images are showing the steps to clean the second connector too.

After cleaning, both connectors are as shiny and as good as new.