Drain, pull thermostat, flush with tap, flush with at least 24 gal. distilled, install new thermostat. Install coolant filter, replace with cat ec-1 elc concentrate. Do not let the water sit for extended period of time. Give yourself the better part of the day to get it done.
This, in essence, worked well for me. Delta still good, if anything it dropped a degree or two.
I also collected some material from my coolant filter, dried it, and have been studying it carefully. Today it was subjected to EDS (energy dispersive x-ray spectroscopy) which shows a lot of carbon and oxygen, with smaller amounts of iron and aluminum, trace amounts of silicon and other things like calcium and sodium in such small quantities that they may or may not really be there, but are on the order of 1% at most. I'm not done with it, but it clearly is not silicate as many have claimed. Nor does it contain appreciable amounts of boron, as I had expected. While it does appear to contain iron oxide, it is in relatively small quantities.
It is completely unaffected by concentrated hydroxide, and by phosphoric/citric acid (the active ingredients in Restore and Restore +, respectively). I cooked it at 115C for three hours, using a higher concentration than the commercial products have in the jug. Took photos and left it on the hot plate over the weekend, got another photo today. It does not dissolve at all, but in the concentrated phosphoric acid the brown color is converted to gray. I attribute this to the conversion of iron oxide to iron phosphate (exactly as occurs with Naval Jelly) but this does not affect the solubility or the cohesiveness of the material. It is unaffected by most common organic solvents.
I have not tested authentic Fleetguard products because I don't have any and don't feel like spending money on them. While they may contain some highly effective mystery material which is non-hazardous (and therefore not listed on the MSDS) it would be a surprise. Generally, things which are perfectly safe are also perfectly useless. I'd love to see someone repeat this with commercial products (or anything else) if so inclined, and am surprised that it appears no one has yet.
I'm not interested in changing the color of the sludge, and what I've seen indicates that the commercial flushing products will be largely ineffective in removing the material that I have isolated; while this may not actually be the case, at this point that claim will require extraordinary evidence. Notice that the manufacturers claim that their products remove silicates, rust etc. but make no mention of this persistent uncharacterized sludge from the 6.0 cooling systems. It appears that the most effective methods will be physical: drain, flush, filter.
For these reasons, my suggestion is to put the filter on immediately. I have not -- and still would not -- flush with chemicals unless there was a compelling reason, and you haven't given any. I also don't see a reason not to switch to an ELC. Southend's advice seems quite sound to me; I would also really encourage the use of the Fumoto valves in the block drains and to use them for every drain. It seems to me that this stuff settles out in the low points; the bottom of the oil cooler, bottom of the jackets around the cylinder bores...