They actually don't stretch, though you get the effect as if they do. What we think is cable stretch is actually housing compression. Housing compresses, mostly when new.

Galvanized means zinc-coated, which doesn't rust, though it does discolor into a dark grey color. Some cables are neither stainless nor galvanized. I agree that stainless are the best, because it stays smooth.

When I install a cable, before I put the cable into the housing, I rub grease on it, AND I pour oil into the housing.