IMHO, Synthetic is simply expensive, I like using a semi-synthetic in my vehicles, it improves gas mileage, returning your money and then some. As for longevity, I need to own a car long enough to find out
Some research, as mentioned, will turn up many confusing answers, so I always thought a semi-synthetic was a safe bet. The 89 octane and K&N really improved my low end response, not sure if it was one or both. Also, as mentioned, because of computers, there is probably nothing to be gained by running a higher octane then the manufacturer suggests, the computer is unaware, or so the story was told to me.