Bernard, if the exterior of the wire oxidizes(it's unclear whether the discoloration Brandon described was oxidation from the air or a chemical reaction with the jacket material, since cutting open even decades old wire usually reveals bright copper inside)it doesn't significantly affect the power carried as long as the connection points are clean. A tight connection tends to be essentially airtight at the actual connection point and any oxidation can be cleaned off with a mild abrasive such as a pencil eraser, or the end can simply be trimmed off and re-stripped.
