Your low end weight number is measured at 100 degrees F. As above, if it gets cold it is gonna start acting kinda like molasses. The flow rate in the engine is gonna slow to a crawl.
True, but to see the actual viscosity of the winter grade you look at the CCS viscosity, cP@ at what temperature. Most 0 grades are tested at -35, 5 tests at -30, 10 tests at -25 and 20 tests at -10. All are Celsius.
You can also look at the numbers even at 104* F (which is 40 C but I hate using Celsius) and you will see a notice a significant change in pumping ability.