The Mobil 1 5W20 ive been using is 8.9cst @100*C and the 10w30 is 10.1 so they are very close to eachother
I agree, they are very close in operational viscosity.
I've used a Euro A3 rated 30 grade oil that had an operational viscosity of 12.2 cSt at 100C (Castrol Magnatec 5W30 A3/B4). So your M1 10W30 is closer to the previous M1 5W20 than my Magnatec 5W30 A3/B3.
Your logic is good, grades are coarse, there is better detail looking at the actual measured viscosity values.
Your 10w-30 in FL is likely the same as running 0w-20 in the northern US tier and Canada
Yep somebody short tripping in a Canadian winter on a 20 grade, is most likely using a thicker oil than I am driving through the outback in summer on a 30 grade or even a 40 grade.
We know cars can handle different oil grades, simply because they can handle different climates and driving styles.