We need to remember that a 6.0L flash has it increasing the idle RPM's periodically, so it will be more fuel consumption than the older diesels.
Also (regarding Ford's position on extended idling):
Page 14 of the 6.0L Diesel Manual:
The following cold weather idling guidelines must be followed:
• Avoid idling the engine for more than 10 minutes at a time.
• Use Motor Craft Cetane Index improvers of a non-alcohol based from a reputable manufacturer.
• Maintain the engine cooling system properly.
• Do not shut the engine down after an extensive idling period (10 minutes or more). Drive the vehicle for several miles with the engine at normal operating temperatures under a moderate load to burn off any accumulated carbon and varnish.
• Consider using an engine block heater.
• For extended idle times use an approved idle speed increase device.
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