Owners Manual specifies:
Diesel Exhaust Fluid (DEF) (U.S.): Motorcraft® Diesel Exhaust Fluid PM-27-GAL / PM-27-JUG
So, if you want to go "buy the book" that would be the DEF to put in. I have just over 4,000 miles on mine now and my DEF gauge is reading 1/4-3/8, so this has been on my mind lately too.
Here is the best pricing I have found on OEM:
If you buy 8 jugs of 2.5 gallons you get just over the $100 mark with free shipping and use Promo Code ALWAYS for another $11 discount brings the total down to $100.33 (after taxes) = $12.54 per 2.5 gallon jug for OEM, with free 3 day ground shipping.
The shelf life on DEF is supposed to be 2 years, and I have historically been averaging ~22,000 miles/year, and figuring 2 jugs every 5,000 miles = ~8 jugs per year, so I just pulled the trigger on this order. I had read some posts elsewhere about non-OEM brands possibly throwing some codes (on Super Duty's), so with this pricing being very competitive to picking up Peak BlueDEF in the local store I just decided to stay with OEM.
I previously also ordered this OEM nozzle assembly to attach to the 2.5 gallon jugs to prevent overfilling the DEF tank and spilling over:
I have read that this DEF is caustic stuff, so take precautions to not spill any on your paint. I am planning on taping up a garbage bag on the side of the truck to protect the paint on my first fill attempt which looks like will be later this week when the product is delivered.
Hope this helps, and thanks a lot for the great question. I was going to go out today to Sams Club and pick up some Peak BlueDEF (which is top rated stuff--> https://bestreviews.com/best-diesel-exhaust-fluids
) but now after doing more research to answer your question will wait on the Summit Racing delivery of Motorcraft OEM DEF. I figure why not use OEM if it will prevent getting some codes thrown over the DEF non-OEM brand put in (of course, this is probably rare to happen), and also no worries over warranty coverage since using OEM.