Oxidation seems to me like one condition that will reasonably (and negatively) effect the way a signal is passed. Oxides are going to be poorer conductors, so as the oxidation spreads, the effective gauge of the copper conductor will shrink -- making a 16ga wire turn into an 18ga or 20ga wire, for example (sort of like closure in galvanized water pipes). 11ga cables are pretty heavy to begin with, but if the oxidation is too significant I still might think about at least tossing some 12ga Home Depot wire in to see if that makes any difference.

Soundhound makes a good point about building wiring. There are a lot of older buildings with power and telephone wiring in use that is decades old. When we do major building renovations (gut back to the shell and start over), we do sometimes call for the old wire to be pulled out, but even in buildings from the 50's or 60's that's not always done. It's more to make it easier to upgrade to newer panels, more numerous receptacles, and modern data cabling. Even after 40 years or more, the wiring should still function (barring pests gnawing through insulation and creating fire hazards).

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