Originally Posted By: gfh77665
Why is "7.5" scoffed at, but 16, 8, and 4 are OK?
Well the first thing, assuming we are talking about the other thread, is that the 7.5 was 7.5W, putting it on the left side of the W, meaning it falls under being defined by the CCS and MRV limits not a range of viscosities like the other grades you mentioned.
Secondly, its got a .5 going on. You'll notice none of the classifications for either Winter or SAE grade in J300 have fractions as part of them.
The number on the right of the W, unlike the one to the left, does actually have a relatively narrow range of viscosities that define it. If the 20 range had been split by 5's rather than into 16, 12 and 8, we would have xW-5, xW-10 and xW-15. I assume that the similarity to the Winter ratings and subsequent confusion that might arise from a 5w-10 or 5w-5 is what led to them using 16, 12 and 8 instead, but I could be wrong on that point, as that's speculation.
Regardless, as J300 clearly shows, each of those designations not part of the Winter rating scheme has a narrow 100C visc range that defines it as well as an HTHS floor:
You can also see that the 100C bottom range for the 5W and 0W designation are the same, and both are lower than the new SAE 8 grade at the same temp. Since the vast majority of lubricants wearing the W designation are multigrades, it is the CCS and MRV limits that are the focus here.