Petroleum oils create sludge and varnish on the inside of engines. This is highly unlikely, but possible.
Most synthetics are very high in detergent action.
They can break loose large particles of the sludge and varnish your petroleum oil left behind. These large particles will circulate throughout your engine, potentially clogging oil galleries and ports. No matter how good an oil is, it does you no good if it can't get to all the places it should in an engine. This is done while the engine is idling so that there is less stress on the engine while the cleaning action occurs. One can per vehicle is sufficient.