But the same is true of a 5w20. It tends to be very stable with temp, just like 0w40 oil. The 0w40 is just thicker all the time.
Magnum you were on the right track up until this point.
The 5W-20 grade is not the most viscosity stable grade; considerably less so than the 0W-20 grade, and the 0W-30, 5W-30, 0W-40, 5W-40 and 5W-50 grades.
This is a better way to look at it. The 0W, 5W, 10W etc are viscosity ranges at very cold temp's; -40 degrees, -35C, -30C etc.
At hot oil temp's, the second number of the SAE oil grade applies; a 40wt is heavier than a 30wt which is heavier than a 20wt which is heavier than a 16wt oil and all those grades have viscosity ranges within themselves.
At less extreme cold temp's, the biggest factor that tells you how light an oil will be on start-up is the oil's viscosity index. For example a 0W-20 oil will be lighter at normal start-up temp's than a 5W-20 because it has a higher VI.
If a manufacturer spec's a 0W-20 grade vs a 5W-20, it has nothing to do with the fact the oil will still pump at -40 degrees and the 5W-20 will not, but rather that it will be lighter at more normal start-up temp's. And the sole reason is because the OEM 0W-20 oil has a significantly higher VI. It's just happens that in formulating high VI 20wt oils they wind up with a 0W rating.
Yes I know it is confusing but the SAE grade will only tell you so much and there is a lot of overlap when you start looking at individual oils.
If you really want to know the viscosity characteristics of a particular oil brand, you have to look up the actual PDS spec's from the companies website.