Can you engine flush to eliminate sludge?

Wow, The engine in question must be very old! Engines produced since around 1960 have been designed to run on detergent oil. Before about 1960, the predominant engine oil approved by engine manufacturers was non-detergent. The oil lubricated well, but was not designed to remove contaminants from inside engine surfaces. Manufacturers felt it was better to let these contaminants ( carbon, iron, copper, lead, acid, water, dirt, unburned fuel) settle into the bottom of the engine rather than allow them to continually circulate through the engine. They believed that it would rapidly wear out the engine. All that eventually changed though and manufacturers began requiring detergent oil. Detergent oil has high solvent qualities; if you doubt this, get some exhaust soot on your hand and then put detergent oil on the soot just like hand cleaner, rub it in well and wipe your hands on a clean rag. Clean, right? All this to say that if you have significant sludge deposits in any newer engine-----then someone has been running the wrong oil in it. If you try to flush sludge out of an engine that is heavily contaminated, the resultant mess will be picked up by the oil pump and sent throughout the engines most close-tolerance spaces and ruin the engine. If the engine is somewhat simple, such as lawnmower, they sometimes acumulate sludge for a different reason; they are air cooled and run at various temperatures depending upon ambiant temperature and engine load. After about two hundred operating hours they will begin to develop a dark, almost baked on film. As the engine gets older the acumulation worsens. It won't interfere with engine operation though other than slightly elevating the running temp of the engine. Some shops now have a machine they use to "flush" an engine if necesary. It depends on how reputable the shop is when determining whether your engine needs flushing. DEFINATELY get a second opinion!