Running at too low a power setting for extended periods is asking for trouble. These engines were designed years ago when tolerances were less exact and are designed to be run at moderate power setting continuously. I have about 700 hours in PA32 types - nearly all para flying and we never babied the engines. The main thing is avoiding sudden temperature changes we never lost a cylinder in 10 years I was operating them - the Cherokee 6 had 3500 hours on it when I stopped flying it. I would not run an IO540 at less than 23/23 for long periods - lean to peak then richen 50 degrees, you won't get too much trouble at that. Also check the prop notes, you may be in an avoid area at such a low setting.