Synthetic oils are great. The friction is a lot less inside the engine, so there is less wear on the parts, lower temperature, better power, better gas mileage....etc. It also gets to operating temperature faster and is not as thick when it is still cold. The problem is that in any motor that runs with normal oil, there is carbon buildup inside the engine. The more miles, the more. If you put synthetic oil into a high mileage motor that always had mineral oil before, these residues can break free and ruin your gaskets, seals and even pistons, valves etc.