The DEF system is a awesome money maker in the parts department when someone puts diesel in the DEF. You have to replace every component of the system that touches the fluid, and Ford refuses to cover any of it under any sort of customer assistance program. Last one we did was around $10,000 customer pay.
What really sucks is when a customer puts DEF in the diesel. On the new 6.7 there is a kit Ford has for that purpose that includes new pump, lines, injectors. Parts alone is around $12,000 and again no assistance is provided to the customer.
FWIW the 2 caps are clearly marked Diesel and DEF. The Diesel is green and the DEF cap is blue.