I think these trucks are misunderstood, truly... And not in some psycho babble sort of way.. but literally.
Check out backglass 6.4 bible.. as far as free resources, it's one of the best. You'll learn all kinds of stuff about these things, and comparing the differences to other power plants both newer and older, you can see where the engineers were really onto something and where they whiffed...
For instance... Oil coolers... A brick in the valley that has oil passages adjacent the coolant, and which is intended to cool the oil to the transfer of coolant... So... A warmer, too, as it is... But there are some functions in that thing that aren't talked about much, here, that are significant by my reckoning. A delta between coolant and oil temps presents itself and people automatically resign to 'clogged oil cooler', and, they may be right.. They may be wrong, too, and simply don't understand that there are no water jackets on the bottom end, as in any motor I know of (auto, anyway) and oil is the crank, rod, camshaft and associated bearings only way to cool- keeping friction to a minimum as the first line of defense, but also that the oil is fastracked when a certain psi is achieved, indicated by high rpm or load, and the cooler becomes pretty much useless during those events... Many a garage or dealer is happy to exploit this rumor, and announce 'clogged cooler, yo, lay down 4g's'.... Lesson? Lifetime coolant and coolant filter w/o silicates... Use good oil with high sheer strength... Don't introduce anything that increases pressure in the system, but DO install bypass filter capable of removing sub 5 micron particulates without disturbing flow or pressure, and remember, when under heavy load the full flow filter is bypassed, anyway, so when you're going to encounter high demand you'll at least have cleaner oil to do its job on standby.
Similar is blow by.. why do these engines produce so much crankcase vapors? They've a monster compound turbo sitting on top of them... That's why... Running the vapors back into the intake cokes turbo wheels, collects in intercoolers creating pressure points and reducing flow, and ultimately slips oil into the combustion which spits remnants at cats, shortening their life dramatically when the zinc (zddp) chemically reacts with the catalyst... Which creates excess back pressure, which is bad by itself but horrendous during a regen- coupled back with the clogging intercooler disallowing air the pcm expects causes excessive heat that only gets hotter when it meets the resistance of a clogging cat, and pistons, valve seats, and valve guides take a beating, allowing more oil past in the short game, and weakening rings in the long game, and compounding the issue exponentially leading up to catastrophe.. lesson? Ccv mod. Reroute at least through a catch can before reintroducing it into the intake, better to atmosphere... Turbo seals will appreciate it, your intercooler and boots will appreciate it, your air to fuel ratio will appreciate it (along with your passenger rear quarter panel and your local popo) your cat and dpf will appreciate it, and they'll extend that appreciation to pistons, rings, valves, valve seats and guides, rockers lifters and push rods.. see? A happy family.
Fuel is to be clean. Clean I tell you.. and lubed... To go from 500ppm to 15ppm, the sulpher has to be removed... Guess what else gets removed in that process? The properties of lubrication. Other than the delicate high pressure system and the house of cards supporting it with the flimsy oe lift pump, fuel also lubes and cools the high pressure pump... Water will destroy everything in the system in just a few passes... Gotta keep it dry and lubed, and filtered as fine as possible.. lesson? Air removing lift pump with sizable water capturing chamber, as good a filter system as you can afford, good fuel to start with, regularly scheduled water draining habits, filter changing habits, and some sort of warning if you lose pressure from your feed (lift pump) to your bowl so as never to turn the high pressure pump dry ever... Ever.. one running out of fuel at high load/rpm and you're likely done with that hpfp. Not a happy day.
I could go on, but it's late... This truck is a tinkerers dream- replacing upgrades with maintenance, and do it religiously and following the advice in many threads here, and it'll treat you very well.. let it go and it will fly apart. Finding one in good shape is getting harder these days, as they're highly prized for their performance potential- they're put up wet more times than not, so to speak. Find one in good shape that's taken care of? You've got a hellhva rig. Put it this way: I wouldn't pay $15k for a dressed out lariat ultimate on a car lot, but there are several people here that I'd pay upwards of $30k for theirs even if it were an xt... That's not an offer, folks... But it is to say that if I ever sold mine I'd hold out for someone who understood the value of a well maintained 6.4 and willing to pay for it... They're worth every penny if they've been taken care of.