I suppose the gauge could be when the 6.4 starts to sound like the 7.3 (what a diesel should sound like) then you should check the oil.
I guess this will just be one more reason that I change my oil at less than 5000 and 5000 being the far end of where I like to be. I really like to start grabbing parts for the oil change around 3000. Oh well, Ford Engineering at its best.