Here is a similar thread with links to other threads on the same topic. This thread is specific to going the other way but many of the linked threads are pertinent as the principle is the same:
Using the new blend or above will be an upgrade. There is no problem with compatibility. While you're at it looks for the Ford F1 recommended oils. Delvace Extreme, Rotella, & several others "Meet" Ford's spec. The delo article was probably their "Isosyn" marketing. If it was so good then why did they discontinue it to start using a synthetic blend? LOL
But that was probably mostly CJ-4 oils that Ford was ok with. Now that API went to CK-4 Ford pushes for their spec'd oils to pass their upper valve train wear tests on the 6.7 and backspec'd all Powerstrokes.
Well, Valvoline PBX is on the list…
Yes, You're right. They did add that spec eventually. There are quite a bit now offered with that spec these days it's hard to keep track. Mobil 1 Full synthetic oil is probably the last of the 6.0L worries so you did well on your choice. I just replaced my FICM with a Dorman the other day in the driveway. Injectors were all reading low voltage. Prime symptom of FICM. I just bought a lifetime warranty Dorman from Oreilly's. Anyways, Thanks for the update.
There are a number of ck4s they certify for their 171 spec. https://www.fcsdchemicalsandlubricants.com/additionalinfo/dieseloilsWSSM2C171F1.PDF
Which shows there's no good reason to not use it & availability is not an issue.
I’m running a supported ck4 (castrol crb 10w30) instead of what they usually recommend in India which is 5w30 a5b5 whether for their small hatchbacks or for a 3.2 liter endeavour aka ranger. Works beautifully, rather better than a thin 5w30.
Sounds like you have a great machine that should serve you well for some time on that Castrol CRB.
