There are a few reasons to use synthetics and ways to make sure your getting the most out of it.
To answer your first question do you need to flush. Only if you have a high mileage motor (100K+) or you have not changed the fluids as often as you should have with dino. In cold climates synthetic will be good for start ups and coating the engine with trucks that tend to sit all winter and only get started then idle to keep them in shape. If you want to truely get your moneys worth from a synthetic do Used Oil Analysis to take the guess work out of when the oil is sheared down and the additive package is toast. With a 7.3L you will get much more life from a synthetic than with a 6.0L.
Synthetics will handle the heat of towing better than dino only because they are more stable and actually run cooler because there is less friction to a synthetic oil.
To learn more about synthetics you can research on the internet at many places. I could give you a few links but remember there is not many places to get what most call unbiased facts. There are test results I could link you with from independent labs if you would like they are on the Amsoil site and some will say they are biased because they are from Amsoil's website. If you look at many of the places and read the test results look at the warranties offered by each company and ask questions you should be able to make an informed decision on what will be best for your situation and your pocket book.
If you decide on synthetics give me a shot at saving you some money on it. Even if you don't go synthetic in the engine I can get you a top flight tranny fluid and gear oil for your rig.
