Superficial oxide will not alter the skin effect or the resistance in such a few meters of this 12 AWG/2.05 mm section cable, such as to hear any difference in sound when compared to the cable without oxide. It does not worry me.Resistance is easy to measure and this shows if the music is affected by comparing resistivity of copper. Besides that really bad oxidation might make stranded copper more like the litz wire, and would reduce skin effect. A good thing according to the audio mystics out there who do not care about factual investigation.
1000 feet = 304.8 linear meters = 1.62 Ohms.
Copper Wire - Electrical Resistance vs. Gauge
Gauge, weight, circular mils and electrical resistance in copper wire.