I've got an OnGuard U-lock that's been sitting outside locked to a rack at work 24/7 for about five years now. A couple of years ago it was starting to bind, but I sprayed some oil in there and it's been fine since.

A U-lock is mostly a deterrent. If someone has the tools to break an old and weak U-lock those same tools would probably break a brand new U-lock a few seconds later. I wouldn't worry about it.