so lets say we let our diesel idle for 8 hours.
it is entirely possible that at idle your engine will not produce sufficient heat inside the combusion chamber, and wet diesel will start to be formed in the combustion chamber walls, and actually produce emissions out the exhaust pipe.
this can cut the oil on teh walls, gum up the rings, and cause the oil to get diluted with diesel fuel.
now whether that starts in 5 minutes or 3 hours is up for debate. the ford PSDs all have warm up programs and an AIH that try to keep this to a minimum.