Quoting: crmalcolm
Your still looking at the cold rating (15W) alongside the hot rating (40).
As you say, the 40 relates to characteristics at 100deg. Now ignore the part after the / and look at the cold rating in isolation.
A 15W oil will start to thicken at higher temperatures than a 5W.
The first part of a multigrade is a rough indication of temperature needed to remain thin enough to use, the second part is an indication of viscosity at operating temperature.
Would it make more sense if you replaced the W ratings with letters.
Eg
0W=A, 5W=B, 10W=D, 15W=D etc.
A/40 oil and D/40 oil are both the same at operating temperature.
B/30 and B/50 are both the same as ambient temperature drops.
A/40 will be less viscous in cold conditions than D/40.
B/50 will be more viscous at operating temperature than B/30.
I can see how the scales can be confused, but on both scales the lower the number the thinner the oil.
Think of temperature, both fahrenheit and centigrade can tell you how hot something is. But 82F is colder than 30C.