So much misunderstanding ...
The "requirement" is for CJ-4 qualified lubes. Period. See this official delivery document; see item #2:
http://www.fordservicecontent.com/Ford_C...ldt_04-2014.pdf
The allowed grades are here, right from the 2015 manual, page 44:
http://www.fordservicecontent.com/Ford_C...-US_02_2014.pdf
10w-30
15w-40
5w-40
0w-30
0w-40
Although not specifically mentioned, I cannot fathom how a 10w-40 would not be considered acceptable here as well.
The point? Use a CJ-4 lube available in about any common grade for HDEO. That is what is "required". There is no necessity whatsoever to run a "semi-syn" or a "syn". There are admittedly some grades that you'll only find in a semi or full syn, but that base stock is not a "requirment" by Ford.
Being in TX, I cannot image that cold winter starts (even in the panhandle area) are a big deal to a common rail modern engine, so I'd think just about any decent brand CJ-4 would do in either 10w-30 or 15w-40 dino.
Severe service mentions the use of "5w-40". They don't state if PAO or grp III is an item, so we might conclude it is not or they would have mentioned it. I find this a bit absurd; I've not seen data to substantiate such an expensive lube is needed for the BROAD range of "severe" conditions they state. By their definition, one would wonder if anyone would qualify for "normal". It is VERY EASY for Ford (or any OEM) to push the "need" for expensive fluids to the customer. The OEM easily can hedge their bets by just letting you pay for syns. Until I would see specific data, I'm not convinced that a syn is required for conditions they state.
If you "want" to use a semi- or full syn, there is nothing that would preclude it, except for wasting money in OEM OCIs.
So far, the 6.7L Fords have not shown any great affinity for any partciular grade, nor any disdain for one either. The generally have run rich with fuel at times, and no base stock is going to stop that. If you've been happy with Rotella 15w-40 up to now, there's no reason to think it would be any different going forward.
If you want to know what is "best", then you need to do this:
1) define what "best" means to you, and what it does not mean to you
2) test many products; this will be a LONG, DRAWN OUT process far past the devotion of most any BITOGer
3) review data in both macro and micro analysis
4) select product which has most pros, least cons, pricepoint you're willing to pay ...
Otherwise, just use a CJ-4 in one of the grades and worry about something else.