Keep in mind, the number in front of the W (Winter rating) is an indication of the oil's ability to be below two threshold numbers measured at two temperatures 5 degrees apart from each other. It's not an indication of the oil's viscosity above those temperatures.
Example:
A 0W-40 has to meet the CCS (-35C) and MRV (-40C) requirements for the 0W-xx designation, which, if you look at J300, are 6,200cP and 60,000cP respectively.
View attachment 84112
Now, a 5w-20 doesn't have to meet those same requirements, and in fact, because of it carrying the 5W-xx Winter designation, we know it's thicker at -35C than 6,200cP, otherwise it would have passed and been labelled a 0w-20.
BUT, at some point, perhaps one not too far above -30C, that 5w-20 is thinner, and gets continually moreso than the 0w-40.