Probably your crappy California "Green" Diesel fuel
Low fuel pressure and fuel contamination (water, gasoline, etc.) are the two biggest killers of diesel fuel injection systems.
So, after verifying sufficient clean fuel, fuel delivery system checking is the next big step. Fuel pressure should be between 3-8PSI, I like to see it a little bit on the higher end usually... when you're dealing with fuel pressure that low, a couple extra PSI isn't gonna kill it, or cause much in the way of driveability concerns.
Synthetic oil wouldn't play much in a part in a fuel system failure... these are common rail and not run by oil like the previous Powerstroke's HEUI system. I am neither here nor there on synthetic products, I know it cost me over $100 to buy just the oil to change the oil in my rig, so I stick with conventional, and change it every 3k miles. I was changing it every 3k miles with the synthetic as well, and I put a bagload of miles on my truck, so it was getting pretty pricey.
The water seperator system on these trucks is piss-poor, at best. The updated fuel filters (FD-4617 I think is the number) have an o-ring land in the bottom of them, that seals the filter to the HFCM, to prevent water/debris ingestion. The HFCM will suck unfiltered fuel/water past the filter when the filter gets plugged up, or the water trap gets over-filled. There's not much room in the HFCM for water. In my personal opinion, I think an Airdog is a good buy, for even a stock truck. It's pretty funny, actually. Airdog is sold in the Ford accessories catalog you can get at the dealer. It ain't cheap, however.
Some stuff to think about, anyways. Being as how it's 4:12AM, I don't have a whole lot more to add at this time. Feel free to PM me if you have any questions.
Welcome to the site, by the way.