Unless you go where it's extreme cold and must have synthetics for cold starts then use regular oil.
You need to change it every 5000 miles.
Saying how great synthetic oil is don't cut it. You have to do a lot of tests on a lot of engines under a lot of conditions and take them all apart. This has not been done to my knowledge on the 6.4 or any other engine. They all use some bizarre form of testing that has absolutely nothing to do with the oil in your engine.
Your regular oil is good for just as long as any synthetic oil is in your motor. The oil will be contaminated if you leave it in too long and it does not matter what type it is.
Unless your going to do oil samples to determine change intervals you need to change it every 5000 miles.
If however you drive only highway and never idle or have short drives or drive in extreme cold you could probably go closer to 10000 miles on either type of oil.
There have been a lot of people that have had problems with synthetics in these engines. Weather it's true or not is not worth the risk for me. I use Motorcraft oil and "my truck runs great".