Look at the bare cable- if it's remotely shiny and light-colored, it's not oxidized enough to matter. I it looks like an old coin, cut off about an inch, strip it and insert it in the terminals.
Inside of the insulation, oxidization doesn't matter, but if the wire looks green, replace it- that's a chemical reaction between the insulation/jacket and the Copper and eventually, some of the strands will break, losing connectivity. Old wire with the clear PVC insulation often turns black or green and once it turns black, it'snot very easy to clean it, especially when the ends are in a place where accessing it is difficult.
Cable price has increased drastically- in about 2008, it more than doubled because the Chinese were buying as much scrap metal as they could get, in order to build the facilities for the Bejing Olympics.
No, I don't want to see anyone tell me that I'm being racist, look into it for yourselves.