And what I am trying to tell you is that it is not.
If two different oils have a viscosity of 70 at 40 C...
This does NOT guarantee they will be the same viscosity in freezing temps.
It would have to have the same viscosity index also (rate of change).
For instance, you listed 70 and 88.7 @ 40 C.
Now here is a listing of a few SAE 30 oils at 40 C
Castrol = 82.6
Valvoline = 86.45
Havoline = 80.8
Now Mobil 1 0w40.... 78.3
Now since we already know that SAE 30 is not safe to use in freezing temps....
And it doesn't get better than 0w...
Well, I guess you can see why you can tell nothing from the viscosity @ 40 C.
And the people telling you your two oils 10w30 & 10w40 have the same viscosity at cold start up... well, they are pretty much correct... especially from the same brand.
Bottom line is that it's the same base stock... but with more VII's in the 10w40 to keep the viscosity the same as an SAE 40 at 100 C.
In fact, most 5-10-15-20-25w's are going to be similar to another 5-10-15-20-25w at cold startup.
Here is the SAE's J300 they use for "w" classification.
That one is a few years old and it may have been updated, but you get the idea.
And from that, it is hard to tell how cold you can go with each.
All you can really tell is that a smaller number is better.
This is why I check consult my manual and also do freezer tests.
Tells me more about actual condition of the oils in freezing conditions.