Owners Manual specifies:
Diesel Exhaust Fluid (DEF) (U.S.): Motorcraft® Diesel Exhaust Fluid PM-27-GAL / PM-27-JUG
Specification: WSS-M99C130-A
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:
https://www.summitracing.com/parts/mof-pm27jug
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:
https://www.amazon.com/gp/product/B008B5WVF8/ref=ppx_yo_dt_b_asin_title_o08__o00_s00?ie=UTF8&psc=1
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.
@1oldengin 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.