Oxidation on surface of cable under clear jacket

I have 30+ year old cheap 10 and 12 awg speaker cables lying around, clear insulation, not a hint of oxidation, and I had some cheap 18 awg that was maybe 10 years old that looked like it was outside from day 1.

My understanding is the material and quality of the insulation determines how much oxygen gets in, which seems pretty obvious, but also good OFC should oxidize slower.

Quick Google says the oxide probably 10's of nanometers thick. I don't see that making a big difference to the conductivity. I would be more worried about the connectors. If they are soldered they will be fine. If they are a good quality crimp they should be okay too. Anything else could be trouble.

Seen it worse than this ....

Winoguy17 yup Mine are worse than that, but that’s it!  And I think as your picture shows, the clear sheathing may be pulled over the copper conductor which allow air infiltration and oxidation to take place.  But when I set up my system in our new house (another post for another time!), I’ll give the Oval 9s and the Kimber 12 TCs a listen to see which I prefer.

So does changing the coating/cover to opaque solve the issue, or hide it?

Seen it worse than this ....

That looks like it would provide limited protection from ingress of external air, but even if it did, there is a lot of air trapped inside to interact with the copper.