i bet it is well below .5 gal.
our tractors have gauges that allow us to read fuel gph (and they are pretty accurate). and with 6.8L displacement, when idling it is reading .2-.3 gph. which come out to .94L, so i would say that skills_kills is pretty close.
idling for 9 hrs may not be good for egr, but beats the damage from coolant becoming frozen. but high idle mod is a definite plus in cold weather.
2004 F350 CC, 35 toyo's, arp studs, scangauge2, coolant filter, aero-turbine muffler, magnaflow exhaust, blue spring update, rhino lining, b&w ball hitch, fumoto valve, tank harpooned, high idle mod.