I find it ironic that Ford (and other OEMs as well) state that "severe" includes things we'd normally associate with a truck, especially a HD diesel truck.
Watching any commercial for any light-truck brand today, you'd swear that these things are invincible; they splash through mudholes, pound and bound over dirt hills and dales, yank schooners from sea-ports, haul cement sewer boxes 6' in height, etc, etc ... So if that's how they sell 'em, why is it "severe" to use them in that manner? And if that's what they expect service to be like, then why would that not be "normal" to be used in a "severe" manner? Would not "severe" be "normal" in terms of how they are designed? It's all marketing hype for sales and CYA in terms of warranty liability; they show you want you want to see to make you feel good about the purchase, then they tell you what they want you to know AFTER the sale has been made, so they can protect their wallet at the expense of yours. This is not unique; it's pretty much the approach every OEM makes for nearly every product you'd see on TV. The mentality is "Ours is the biggest, baddest, best in the market" and then you have to pay to maintain that product with premium products they recommend at your own expense.
Which brings me to a topic we've discussed before. There is a BIG difference between what is "recommended" and what is "required" in terms of product warranty. They may "recommend" 5w-40 for severe use, but they "require" a CJ-4 certified lube. This is a distinction that goes unnoticed by many. You are obligaged to use a CJ-4 product for the warranty, but you are NOT obligated to use any particular grade. This is all about the concept of burden of proof when it comes to the Magnuson/Moss warranty act. See and read all the info at the FTC website:
http://www.ftc.gov/tips-advice/business-center/guidance/businesspersons-guide-federal-warranty-law
Looking over the 2015 diesel supplemental document, I see that Ford has the following grades as acceptable for use:
http://www.fordservicecontent.com/Ford_C...-US_02_2014.pdf
page 44 shows these as available choices:
10w-30
15w-40
5w-40
0w-30
0w-40
All depending upon expected temp ranges and use. That's a darn broad brush to be painting with, if you ask me. And since many of those "severe" conditions cannot be mitigated by grade, what exactly is their basis for any particular selection in the first place???? Phooey!
In that document, page 57, it shows that all but the 0w-30 meet a Ford spec of WSS-M2C171-E. It used to be that Ford had a different spec for differet grades.
But four of these are all rated under one spec! So how important is grade if one spec covers all of them?????? See this other BITOG thread
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1220093&fpart=1
There are plenty of lubes that I can find that meet that fairly open spec, from many different sources.
Look these over; these are Ford M/C oils that ALL are to that same spec, but of different grades:
https://www.fcsdchemicalsandlubricants.c...ory=Motor%20Oil
https://www.fcsdchemicalsandlubricants.c...ory=Motor%20Oil
https://www.fcsdchemicalsandlubricants.c...ory=Motor%20Oil
If you read the diesel supplement carefully, on page 45, it says this regarding severe service:
The following conditions define severe operation for which engine operation with SAE 5W-40 API CJ-4 is recommended. (they then list the conditions you stated previously in your OP).
Note that they "recommend" 5w-40, but they do not "require" 5w-40. That is because any lube that meets the spec (WSS-M2C171-E of any grade) is sufficient for use. So go grab a CJ-4 lube and be happy!
So, for more than 8 years, Ford has had essentially one spec for their diesel lubes, but they allows different grades within that same spec. Anything that would be CJ-4/SM would meet that spec. That is what is REQUIRED for the purposes of warranty coverage. They only "recommend" grades.
The reality is that I've seen absolutely no evidence that the vast majority of engines have any real preference in terms of WEAR RATES when it comes to viscosity, as long as you're reasonable in your selection. Going up or down a grade makes no real difference. Choosing an appropriate API category is important; I'd not use a CH-4 where CJ-4 is required for DPF, EGR, etc. I'd not use a SJ where SN is called for. But going backwards is certainly OK.
I will note that the HEUI systems do seem to prefer a lighter grade in terms of performance, but that has nothing to do with engine wear.
Overall I see no reason whatsoever to not use the 10w-30 year round in your 6.7L, regardless how it's used. However, if you're trying to find a "one-size-fits-all" lube for your diesel fleet, then you'll have to make a compromise somewhere. I would expect that 15w-40 dino HDEO would be fine in your 6.7L as well. You may lose a bit of fuel economy, but you'll not be risking any wear issues, and with HEUI out of the picture, that's one less thing to worry about in terms of vis.
I use 10w-30 dino HDEO in my old Kubota, my Dmax, my 1966 Mustang, my Scag air-cooled Kaw motor, etc. It's a great general all-around product. If you're that concerned about trying 10w-30, you can always does some UOAs on your older equipment with the 15w-40, and then try 10w-30 and do UOAs to see if you find any significant shifts in wear trending. Or, go the other way; keep the 15w-40 in your old stuff, and try 15w-40 in the 6.7L PSD. Do UOAs in that direction.
Every experiment I've done, along with tons of data I have, indicates to me that 10w-30 HDEO will suffice anywhere 15w-40 will work and vice-versa. But if you doubt my testimony, then by all means prove it to yourself! Nothing wrong with assuing yourself with your own experimentation.
