Petroleum oils create sludge and varnish on the inside of engines.
At the very least, you will suffer oil consumption due to debris which is cleaned out by the detergent additive package of the synthetic oil. Most synthetics are very high in detergent action. They will break loose the sludge and varnish the petroleum oil made in large particles. 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.