A shop that has loads of 6.0 experience told me this.
EVERY 6.0 engine that came in with a blown head gasket, had a radiator cap that had failed, and the cause for the blown head gasket WAS the bad radiator cap... He explained it this way:
Coolant pressure raises the boil point of coolant, so a 16 lb cap lets temp of coolant in radiator rise to 280 degrees before it boils. Cooling fan clutch is built to kick in at, say 230 degrees, and cool radiator...
Cap pressure failure does this.. it lowers the coolant boil off temperature, to a point BELOW the fan clutch kick in point, so at, say 6 lbs pressure, the boil off temp lowers to, say 218 degrees... the fan clutch does not kick in the fan... coolant boils off, less coolant causes bad cooling at head and cylinder walls... temp rises higher... more coolant boils off... spews out the Degas tube overboard as steam and hot water, which exits below the sight line of the driver ... engine cooks.. while the fan clutch is waiting to reach it's "kick on the fan point".... but the coolant boils off before it ever does.
His advice.... Replace the radiator cap [degas cap] at EVERY oil change interval... and ONLY USE Motorcraft caps... New caps cost less than $8.. cheap insurance.
I immediately tested the radiator cap on my 2005 F350 6.0 dualy.. which had been losing coolant around the cap [look for residue] ... sure enough... it was bad... Ford, in it's wisdom to save pennys, uses the same cap of a bunch of it's small engine cars and trucks... it's not up to the task of the diesel system... Use it.. throw it away... put in a new one... OFTEN. Every oil change is now my rule.