You'll get opinions both ways and probably some truth in both positions. Modern oils do tend to clean out the sludge but that can stop up some valve oiling tubes, maybe even clog up a pump. My advice would be to pull the side plate and valve cover and have a look see before doing anything. If it's really bad then maybe it should be taken down for a thorough cleaning. At a minimum that will allow you to clean out what you can see. If you start changing it I'd forget the diesel fuel idea in favor of 20W detergent motor oil and get it good an hot too.
Just curious but how does it sound after 23 years?
