Consumer Reports did a test on motor oils years ago. There was a lot of analysis, but the end conclusion was that synthetic oils were no better than most regular motor oils when used in an automobile. They said that it was better to change your oil and filter per the auto manufacturer's schedule than to extend the interval and use higher priced synthetic oil. Also, the oil filters clog up. Once their clogged, the oil bypasses the filter element, which just circulates the dirt. It's much cheaper and better for your car to use regular oil and change it more often.
Synthetics last longer in jet turbine engines and exotic high performance racing engines, but they do nothing for normal cars.
The best oils were not a big surprise: Valvoline, Quaker State and Pennzoil. Skip the additives, and NEVER USE RECYCLED OIL OR HAVOLINE! They didn't even recommend putting Havoline in your lawn mower.