I work at a local ford dealer, we had one of out good customers come in the other day w/ a bad lack of power on his "04 6.0L... exhaust smelled really odd too!!
It had very low fuel pressure, we took a fuel sample was dirty,brown and thick!! almosty as thick as 10W-30!!!!
So we ended up removing and draining the tank as well as all of the nasty setiment from the local chinese restaurant..
replacing the fuel pump/heater assembly as well as the fuel filters.
luckily for the customer this fixed his truck and did not ruin the injectors.
He told us that he had boughten fuel in another state and could not remember the brand or even where it was???
So he did have to pay for this repair as well as some diag time.... THIS IS NOT COVERED BY FORD FOR 5YR/100K!!!MAKE SURE TO HEAT AND FILTER THE OIL IF YOU ARE GOING TO ATTEMPT THIS!!
good luck..