From what I have read on the topic, here is my understanding of conventional vs synthetic:
-- From a strictly "base stock" perspective, synthetic oil does not lubricate better than conventional. If there is an improvement in lubrication (more MPGs, more power, cooler running temperatures or something else), it's the additive package that makes the finished product a better lubricant. Synthetic oils being premium products, they often have better additive packages -- this includes improved detergent and dispersant package, so synthetics tend to keep engines cleaner, and they can clean up dirty engines quicker than doing more frequent changes with conventional.
-- Under equal operating conditions, synthetic oil lasts longer because it's better at tolerating high heat and shear, and generall resists breakdown better than conventional over time.
-- With the same viscosity, synthetic flows better at very low temperatures and thins less at very high temperatures (viscosity stable).
Modern conventional oils are excellent oils and I won't fault anyone for running conventional if it meets the manufacturer's requirements. But considering the small price difference, I think it is better to run synthetic. And when name-brand synthetic motor oil can be had cheaper (after rebate) than conventional, it is a 100% no-brainer decision for me.