The automotive organization ran through a variety of tests analyzing how temperature, mechanical shear, and other variables affected viscosity. It also tested the oils for volatility, which the organization explains could have implications for oil usage due to evaporation, and propensities for leaving deposits and oxidizing. When it came to maintaining viscosity in different temperatures and under stress from mechanical shear, synthetic oils, on average, performed significantly better than conventional oils. Synthetic oils were also less volatile, and they left fewer deposits. The one area where there wasn't any significant difference was in how quickly the oils would oxidize.
These results make it clear that synthetic oils generally have an advantage when it comes to engine protection, so if you're looking to give your car the longest life possible, it's likely a worthwhile investment. AAA does say that the conventional oil still meets standards for engine oil, so is still fine to use in a car. Also, considering the remarkable array of oil brands and their different lines of oils, there is the possibility that a different set of oils would have resulted in a somewhat different results. However, as one of the few independent analyses of the differences between types of oils of which we know, AAA's test should work as a general guide that's more reliable than hearsay.
If you want to really go in-depth with the results of AAA's study, you can check out the full analysis at the company's website, here. Just click on the link to the report, and you'll be able to peruse oodles of graphs and technical descriptions.