The injector problems are definitely fuel related, at our shop we just replace injectors in a 04.5 5.9 that had almost 300k on its original injectors, the truck is serviced regularly and used everyday to haul livestock. That same truck has a 6 speed manual that was replaced with a rebuilt at 180k and by no means does it haul heavy loads. On a side note, we replaced a 6 speed manual in an 08 6.7 cat delete, with a tuner, 4 door single axle, the owner said he saw 50k a couple times on the scale with his 5th wheel hauling large hay bales!
That was an expensive one with close to 6k in clutch and rebuilt trans.