Above freezing temperatures there is little difference between 5W, 10W, 15W, and 20W. There is difference but very little.What is the consensus regarding 10W40 vs 20W40?
The definition of 10W is, "an oil which usefully flows at the same cold temperatures as a 10 weight oil." The W rating is assigned based on the temperature, not the viscosity at a common temperature such as 32°F or 0°F.
To create a 10W-40 oil traditionally a 10 weight oil is used then Viscosity Improvers are added to keep the oil from thinning no more than a 40 weight oil would air 212°F. Note that at 10W temperature the oil has a much higher viscosity than at the 40 weight rating temperature. The oil doesn't get thicker as it warms, it only doesn't get thin as fast as a 10 weight.
There is no definition of viscosity between cold and hot. There is no definition above or below the extremes. Marketers of synthetic leave this to your imagination. "Of course we are better! We are synthetic!"
Viscosity Improvers are weaker than base oils. Fewer are needed for 20W-40 than 10W-40 when using same quality of base oil. This drives the 'diesel" convention of 15W-40, that it should stay in grade longer with a 25 point spread than 10W-40 with 30 point spread.
We have no idea how much VI is used in any oil. Is said synthetic base oils can be made to span common multi-viscosity ranges. But once again we have no idea whether whether a superior base oil was used or VI when oil is sold as synthetic.