I concunr with the assessments regarding stichion and FICM; too many folks confuse one for the other.
It's been a while since I've studied the start-up ops for the 6.0L. Does it have an intake grid heater? I know it has glow plugs. Anyone have any idea how long the plugs/heater are "on"?
This is why I ask ...
My Dmax (understandably and admittedly a different engine) uses both glow plugs and intake heater. The plugs and the intake heater grid will stay on for perhaps a couple of minutes after cycle start. During that heater time, my voltage will drop down to about 12v supply because the grid and plugs are eating voltage and amps like crazy. When the heat assistance shuts off, the volts immediately jump up to around 14.5v, and the dash and driving lights will become "brighter" to a small degree.
The reason I mention this is because if there is a major amp/volt draw in the 6.0L PSD system during that first minute or two, then the FICM is not getting it's full supply voltage. As I understand it, the FICM can compensate a small amount, but it may not be able to fully compensate for the loss of input supply. I have always heard that the 6.0L FICM is dependent upon very strong batteries and alternator to operate at full efficiency. The output of the FICM is dependent upon the input of power from the batteries. The FICM is a glorified high-tech transformer/distribution system that controls all aspects of the injection events (timing, pulse width, etc). In short, consider it this way; garbage in - garbage out. Poor vehicle supply voltage will result in poor FICM output. I am not an expert on the HEUI system, but the various reports I've read from knowledgeable sources lead me to these conclusions.
And I've seen just such an example in the 6.0L of a friend. He allowed his old batteries to drag down the FICM and it was damaged, and even after replacing the batteries, the FICM was still slow and the injectors were "lazy". He complained about the truck not having the same power as when it was "new", and a loss of fuel efficiency. But he never replaced the FICM like I encouraged him to do. Eventually he got rid of the truck. After suffering nearly every other 6.0L issue (related to the EGR/oil cooler), he finally cried "uncle" and got rid of it. It's a shame because after spending so much money on the other fixes, he only needed to do the FICM and he'd been back to full power.
I had convinced him to run 10w-30 HDEO (Rotella TP) and his UOAs were excellent. The thinner lube also help reduce the "romp" effect on cold mornings. But thinner oil cannot compensate for a poor injection control system that has been compromised via negligence.
It's not that I really believe the 6.0L is "high maintenance", but perhaps more accurately described as "maintenance sensitive". You cannot let the little things degrade over time as the HEUI system is just too susceptible.
Using a PAO and aftermarket additive may off-set the FICM being degraded to some degree, but they probably cannot fully restore the loss of performance from voltage drop in the FICM. It may seem "better" after installing a PAO/additive, but it also may not be fully "right" until the source of the issue is fixed. I see the PAO/additive as a crutch; it doesn't fix anything but can help compensate until root cause is addressed.