I did get 2mpg better mileage with Synthetic oil than Dino. This is just my experience. The best way to get good mileage is to keep your oil clean and full. Tires at the proper pressure. And most of all stay out of the gas.
As for how long your motor will last. The most mile I have recorded on a motor was 386k on my 78 280Z. I sold it at that mileage and had no problems with the engine at all.
Just my experience's
Shane