I personally am not sold on the "added benefits" that are associated with synthetics. I have tried them in a few different engines over the years and was never real impressed. It seems you can take an engine that is healthy and doing well on dead dinosaurs and by simply changing the oil to synthetic, you can create alot of leaks and additional sledge build up on the internals. That said I run Rotella T4 with is the conventional version of the aforementioned T6. It seems to keep things a bit cleaner and my engines don't seem to use/lose as much oil between changes.
I would also like to add that this is all based on my personal observations over time and your mileage may vary. Its just an opinion, and we all know what those are like.....