Amp Gauge Wire Diameter (AWG)
This is a 1980s jeep but provides an example of oxidized battery feed in the engine bay.
There was a small break in the wire insulation, and some greenish copper oxide could be seen.
Removed the molded insulation and slit the wire insulation open.

That's not good. Current carrying capicity of that wire is reduced.
The fibre 'pen' and Deoxit can be used for cleaning.
Can it be done repaired. Yes but it is difficult. I did it on the bench.
This one is Chrysler.
Crimped and soldered. Crimping needs an open barrel crimper that can handle heavy terminals for 10 gage wire.
Like I posted before, wire splicing is a bit easier.