This is starting to make more sense.
So I guess that begs the question, why do people (mostly everyone really) stress over running 5W-30 in the winter and 10W-30 in the summer? A lot of people (mid to south US) have "summer oil" and "winter oil".
So referencing ^^^ does 0W/5W/10W even matter in the summer months?
There's no reason to do that. In fact, the 10W-30 variant is likely worse off than the 5W-30. A 10W-30 is easy to achieve with cheap, junk base oils and VII, and since all of the major brands are in a race to the bottom, they gladly exploit that. (with no corresponding drop in retail cost, of course) The people doing that simply don't understand what the oil grade stands for. The winter rating doesn't matter in the summer months. They could just run the 5W-30 year around and likely be better off.
As stated, the first number in a multi-grade isn't the actual oil grade. It's just an extraneous winter rating, hence the "W" stands for winter. It's a representative figure for the coldest temperature at which that oil is still pumpable, measured in dynamic viscosity (centipoise). This does not change with the second number so 0W-20 and 0W-50 oils have the same winter rating.
Per SAE J300
0W-xx = <6,200 cP @ -35°C (-31°F)
5W-xx = <6,600 cP @ -30°C (-22°F)
10W-xx = <7,000 cP @ -25°C (-14°F)
15W-xx = <7,000 cP @ -20°C (-6°F)
20W-xx = <9,500 cP @ -15°C (4°F)
The second number is the actual oil grade based on a kinematic viscosity (in centistokes) range at 100°C (212°F, often abbreviated as "KV100") and dynamic viscosity (HTHS, in centipoise) minimum threshold at 302°F. Kinematic viscosity is often also measured at 40°C (104°F, often abbreviated as "KV40") to get a viscosity index which is the rate at which the oil thickens as it cools, but this is unrelated to the oil grade. Like above, this does not change with the first number. A 30 grade is a 30 grade, regardless if it's a 0W-30, 5W-30, 10W-30, or straight 30. The only exception with this is the 40 grade figures where a 15W-40 must meet a higher dynamic viscosity than 0W-40, 5W-40, and 10W-40. Like the 10W-30 example I gave above, a 15W-40 is dirt cheap to make with bottom shelf base oils, viscosity index improver, and additive chemistry, hence it's held to higher standard in dynamic to at least keep it from being a total dumpsterfire.
xW-20 = 6.9 - 9.2 cSt @ 100°C and >2.6 cP @ 150°C
xW-30 = 9.3 - 12.4 cSt @ 100°C and >2.9 cP @ 150°C
0/5/10W-40 = 12.5 - 16.2 cSt @ 100°C and >3.5 cP @ 150°C
15/20W-40 = 12.5 - 16.2 cSt @ 100°C and >3.7 cP @ 150°C
xW-50 = 16.3 - 21.9 cSt @ 100°C and >3.7 cP @ 150°C
So a 5W-30 oil must be <6,600 cP @ -30°C and fall between 9.3 - 12.4 cSt @ 100°C (plus >2.9 cP @ 150°C). A 0W-40 must be <6,200 cP @ -35°C and fall between 12.5 - 16.2 cSt @ 100°C (plus >3.5 cP @ 150°C).
Note, what I stated above about companies going cheaper on the narrower multi-grades only really applies to the common major brands under the API certification umbrella. The API certs have the side effect of a ceiling on performance with no incentive to formulate the oil to anything more than the bare minimum. Hence, you get junk like Rotella. This usually isn't the case with boutique oil companies (HPL, Amsoil, Red Line, Driven, etc...) as they're more performance oriented, not shackled by API certs. They tend to put as much pride in their 10W-30 and 15W-40 oils as they do their 5W-30, 0W-30, 0W-40, etc...
I also live in SC (Edgefield), and use HPL PCMO 10W-20 in 2 of my vehicles, HPL HDMO 10W-30 in the other 2. Also, HPL Bad Ass 0W-12 in the drag car.