One problem related to diesel engine has been reported for the 2008 Ford F-550. The most recently reported issues are listed below.
I am original owner of a 2008 F-550 superduty used for my business. It has approximately 108,000 and has required the replacement of entire engine assembly twice. Both instances the truck was traveling down the interstate at highway speeds when the failures occurred and the driver was unable to get to shoulder of road. Both failures also gave no warning. First failure occurred in August 2009 @ 32,225 when driver heard a loud noise coming from engine and it immediately stalled. Truck was towed to dealer upon which it was determined a connecting rod had gone through block. There was no explanation provided except the engine was still under powertrain warranty. Second failure occurred April 2012 @ 107,000 while driver was on his way to dealer to get radiator fixed due to a leak. He was on the beltway when he immediately started to loose power, it stalled with a chance of getting to shoulder. When towed to the dealer, it was diagnosed as excessive coolant in oil pan (3 gallons) which caused internal failure to the bearings/crank/rods. Engine had to be replaced again. This time failure was caused by the water pump front cover that was severely pitted allowing coolant to enter engine. This front cover had been replaced around 71,000 for same problem, which according to the mechanic, is common with 6. 4l engine. I have serviced this truck according to Fords recommendations. Have had the oil changed more frequently than 10,000 mile that is recommended for the type of usage. Dealer agreed that 7,500 miles between oil changes was sufficient since truck was not used in severe operating conditions. Concerned for safety of my driver since this truck has stalled twice on road resulting in major engine repairs even though I have properly maintained the truck. I have googled and found many other owners of Ford trucks with 6. 4l diesel that have had same problems I have had. Updated 07/19/12.