True, the wider the range (low number is low temp, high number is high temp), the more polymers are added.
So a 0W-50 oil would have all polymers! 10W-40 is pretty good year-round oil. The "10" means it works well in low temps, and the "40" means it goes up farily well to high temp. But it is still a compromise.
Since most of us change oil frequently, better to choose the right weight for the season/temp, and your driving style.
So for winter/low temps you want a "thinner" oil that will do a good job of lubricating cylinder walls and not get thick/sludgy in the low temps. So I go with 5W-30 for cold areas.
For summer, get something like 10W-40, so the "40" means the oil is "thcker" and will not thin out and possibly bubble under high temps.
For very cold, look at oil with a "0" on the low range number. For very high temps, or track use, look for something with a "50" on the high range number.