The lighter oil will perhaps get to full flow sooner. It will always take less energy to pump at sensible flow rates.
I'll say this poorly ..but ..
The way an oil reacts at those cold spec's doesn't have any relationship to its sensible visc. Don't confuse that with meaning that they'll all reach some sensible flow rate in any equivalent manner ..that your oil pump will not be in varied levels of relief between two unlike viscs.
Assuming that one is indeed in -35 or -40 conditions, I will assure you that when comparing 0w-40 to 0w-20 ..that you will realize FULL FLOW much sooner with a 0w-20. You will always consume less energy with the lighter oil at any point in the whole process between the two oils in comparison. It will always be easier to pump (assuming like cold spec's).
This is PZ PP 0w-20/5w-20/5w-30
MRV viscosity, cP (°C) ASTD D-4684 17,500 (-40) 9,700 (-35) 14,800 (-35)
The 0w-20 is higher than the 5w-30 ..but -5f colder. When comparing the 5w-20 to the 5w-30 ..the comparison is more clear.
Shell 0w-40 has an MRV of 24182Cp @ -40
All of them won't exceed stress limits for their respective designations at the designated temps (CCS).
Here's the CCS spec's for the same 20 grade oils
CCS Viscosity, cP (°C ) ASTM D-5293 4840 (-35) 4,250 (-35) 5,150 (-35) 4,570 (-30)
Here's 5w-50
CCS Viscosity, cP (°C ) ASTM D-5293 4980 (-30)
MRV viscosity, cP (°C) ASTD D-4684 24,000 (-35)
Very few of us operate at these temps. Transitions from motionless to flowing will vary with temp.
Again, I'm lacking in some formal education links ..and the self taught process is challenging. So there are surely a few flaws here.