I emailed Smith & Allen about it last year, they didn’t even bother to reply!

I can now see their point of view, after all the interest even on this informed forum hasn’t been exactly spectacular, so the general population of owners out there would probably be too scared or reluctant, and a bulk purchase could take a very long time to sell.  “If it ain’t broke, don’t fix it” -where’s the fun in that?

Gremlin, I for one would deffo use the 40 grade oil if one could be made.
From all I've heard a 30 grade can quickly deterioate to 20 which is concerning.
You made a very valid topic by starting this post.

Gremlin500 wrote:

@D3Jon:

No, the point of my thread was that I’d found the “Holy Grail”, -a 5W40 C4 that would directly replace the watery C1, yet not f*ck up my DPF, knowing that a C1 5W40 simply doesn’t exist.
The other point of the thread was to gauge interest, before I decide how much of it to buy, assuming the price isn’t silly.

It does seem this whole subject is poorly understood in general, and also that folks are scared to try anything that hasn’t passed through JLR’s approvals process.

What will those people do when C1 becomes unobtanium in a few years?

The short answer is no, at least not in small quantities, only in bulk (like 5 figure sum bulk). I was prepared to go for it, but got a lukewarm reception on here, so didn’t.
My advice would be, keep using the 5W30 C1 ULS, and change the oil & filter every 5K miles, (well before dilution gets significant). If you have a V6, replace the oil pump when you replace the belts.
Maybe things will change as more AdBlue cars fall out of Warranty, so the 5W40 C4 will become a thing. “If it ain’t broke, don’t fix it” -where’s the fun in that?