A lot of things don't make sense until you understand them. This is getting far from the original question in this thread, but here goes.
The winter weight oils have to meet a set of viscosity specifications that are mostly separate from the other weights of oil. The only common viscosity spec is minimum kinematic viscosity at 100 degrees C. Additional specs for winter oils are all at lower temperatures, for non-winter oil the others are all at 100 degree or higher temperatures.
The winter oils each have a maximum viscosity at a specified low temperature that varies by each weight. They also have a maximum pour point/pumping temperature for each viscosity rating.
The non-winter grades do not have to meet any of the low temperature tests but have to maintain minumum shear rate viscosities at 100 and 150 degrees centigrade.
Multi-weight oils, such as 10W30 use viscosity index improvers that allow them to pass both the low temperature tests of a 10W grade oil and the high temperature tests of 30 grade. Straight grade oils do not contain viscosity index improvers.
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