I worked in an auto shop in high school in mid 1980's. Back then the type of engines that'd come in that didn't have oil changes often enough was an entirely different world... Many times I'd pull a valve cover and see the everything inside covered in a 1/2 inch thick layer of sludge, and you really couldn't do much about cleaning it because the whole engine inside and out looked like that. But these days, the amount of sludge you get is way less severe. Engines run so much cleaner these days. And my Prius with 300K miles is still hard to read the dipstick after an oil change because the oil is clean and stays clean for a long time. As in, not changing your oil often enough is way less of an issue than it used to be.
The tolerances were much wider too. Clean out the sludge, you'll likely get oil leaks or run into problems that were not there before you cleaned it out. Most engine covers could be safely cleaned without messing up the rest of the motor. Of course you tell the owner to change the oil more often to avoid these steep bills. It was really funny back then watching an engine guy trying to rebuild a transmission. They could get away with the dirty mess they were leaving in a motor rebuild; but NOT in a transmission rebuild.
