The point I'm making is that certain synthetic 15w-40s we see around here may actually not fall technically within the 15w-40 specification. It's been a bone of contention for several on here, with some of us theorizing that the only reason that 15w-40 synthetic HDEOs are offered is because there are many that are very old school and wouldn't put anything other than a 15w-40 in a diesel, while these 15w-40 synthetics are actually closer to a 5w-40. Others have argued that this is a major violation of API regulations and is not allowed.
A synthetic will certainly beat a conventional oil of the same viscosity for winter specifications, usually, but I have seen the odd exception in some product lines, but even in those cases, the difference was hair-splitting. However, the example I gave is the widest spread I've ever seen between two modern, CJ-4 15w-40s.
In North America, too, you have to remember that our experiences with diesel powered cars has been limited and, generally, less than stellar. GM brought out a few dogs back in the day, and they tended to use 15w-40, since that's simply how it was done then.
On the other hand, VW brought some fine engines to North America, but we had dealers that had zero clue about the oil specifications and tended to use PCMOs in them. I suspect that's a significant reason as to why we continue to have VW diesel owners in North America relentlessly searching for alternative lubes. If the dealer can't service them correctly with their access to approved lubes (which many didn't use in the first place), it's no wonder that people look for alternatives.
If I had a VW diesel and was saddled with the dealer trying to put 5w-30 API/ILSAC GTX or Syntec into the thing, I'd probably be looking at 15w-40, too.
