The the idler pulley and the belt tensioning mechanism are the same thing. Good belt dressing does more than make the pulley(s) and belt sticky it also penetrates the belt to help keep them softer and more pliable. Depending on the layout of the belt tracking the back of the belt may also turn more than just the belt tensioning pulley/mechanism, the back of the belt is usually what produces the squeal. Changing the belt is actually very good insurance against a broken belt. If you don't have a diagram of the belt routing you can always take a picture for reference. As stated the serpentine belts are much more easier to change than the multiple v belts. Make sure the belt idler pulley/tensioning mechanism is functioning properly.