John Baker | Mar 18, 2016 8:55:01 AM The simple answer: Most oil manufacturers recommend changing their synthetic oils according to the service guidelines given in vehicle owner’s manuals. A few synthetic oils on the market, however, provide recommended service intervals on their back labels that are generally longer than those given in vehicle owner’s manuals. Each is different, so check the label carefully. AMSOIL Signature Series Synthetic Motor Oil , for example, protects so well motorists can use it for up to 25,000 miles/12 months (15,000 miles/12 months in severe service) if they choose. The detailed answer: Most motorists now understand that synthetic motor oil performs better than conventional oil. It delivers improved wear protection, offers better engine clea...