Ok I'll give you the limits rating, but it's still a guaranteed max viscosity if you look at it that way, it's for definition of a cold cranking rating so needs a cap on its viscosity.
Yes, it's a pair of maximums, set at different temperatures, depending on the Winter rating in question. Cold Cranking is usually the one that sets the rating, since the limit is a lot lower than for MRV.
Of course, unlike the non-Winter SAE grade, it is open-ended on the other side, there's no minimum CCS or MRV, just maximums.
Thing is, while it is limited by a viscosity
value, the Winter rating itself is not a viscosity (per your initial questioning of my statement). Rather, it's a performance target, defined by CCS and MRV, which of course both have viscosity limits, but these just work to define the bounds or parameters for our grade/rating.
That's why I worded my statement the way I did
I'll also give you the pedantry because that's true and something most don't know about, but at a high level, the SAE number is referring to a viscosity, if only a bracket of viscosity.
The 100C ranges that define the non-Winter grades are definitely a lot narrower, but are still ranges. I like your use of "bracket" there, that's a good description.
The funny part is, if you look closely at J300, you'll see significant overlap between 8, 12, 16 and 20. Of course that whole range used to be 20 before these thinner oils were introduced. 20 has a floor of 5.6cSt, and 8 has a ceiling of 6.1cSt, what sets them apart then, is the HTHS visc at 150C (yet another parameter not captured in the Winter ratings) and why things get a bit weird down in that area.
That's why I think it's important that people understand that these aren't firm numbers; aren't a given and fixed viscosity, but rather a category, range, or as you said, bracket; a spectrum if you will, and your oil could be dead in the middle or on either end depending on how it was blended. The Euro xW-40's tend to be on the low end, while the HD xW-40's tend to be on the upper end. HTHS tends to track that way with those oils as well, with the Euro lubes being closer to 3.5 and the HD's being up around 4.
