Synthentic oil of course is supposed to be more free flowing, and offer more protection. All studies that I have read show that it's really only benificial at the critical points, such as if you should lose all your oil, and keep driving to a shop or something. Which almost no one would ever do. They claim you can also wait longer to change your oil.
High mileage oil is usually thicker, like at least 10W-30, and it's conventional oil with a different combination of additives, to offer more protection and condition seals. Also to form a better seal in the cylinders.
Most people I know have found that conventional oil is just fine. Some claim that a semi-synthetic is the best. I don't know anyone myself that uses high mileage oil.
I have near 117,000 HARD miles on my '95 GA (4 cyl) and still use 5W-30 conventional Quaker State. I still have plenty of "get up" and NO oil consumption.
Tony