Just to calm your nerves, a 0w-30 has identical limits for stress in the Cold Crank Simulator @ -5F lower than 5w-X. The hot visc is in the same range of a 30 weight. The “W” spec is not a visc, it’s a property/characteristic. It means it “will pump” and not fracture and form a gel or crystal. It’s not a rational term for most of us. I don’t have the physics background to intelligently speak on the topic (and no one who has can speak in a manner that anyone else can understand). There is no “0” weight. There is just better than 5W. There’s also no official SAE 5 or 10 weight. While some of you may have seen 10w on a 5 gallon pail of oil, it’s a 20 weight that cannot meet a High Temp/High Shear (HTHS) spec of >2.6 …hence it cannot be called a 20 weight. It’s sold for its cold properties and used primarily as a transmission/gearbox fluid.
0w-30 vs. 0w-40. Both will be below limits @ -35F for the stress on the Cold Crank Simulator (CCS) …but that’s where the likeness goes away. That 40 weight is MUCH HEAVIER all the way through 100C/212 and will always be heavier in any Newtonian state. The two fluid may react with similar results as non-Newtonian fluids.
A Newtonian fluid will be like water in glass when you have a spindle spinning in the middle. It will form a small circle and that circle will drive a bigger one …and so on until you reach a very big circle moving very slowly. This is laminar shearing. The friction of the inner drives the next division. In non-Newtonian fluids the liquid at the center may just spin or climb the spindle, the outer fluid stays still.