a 0W- 20 would at lower temperatures behave like a 0W- 40 and
a 0W-40 would behave like a 15W- 40 at operating temps.
Note that a 0W- 20 would be avfully thin in an engine designed for a xW-40 at operating temperatures.
Also noteworthy is a number called VI or viscosity index, the higher the number is the change of viscosity decreases
due to temperature changes- ok good just get the highest VI or ?
Well this opens next can of worms.
Every baseoil has a natural VI number wich the blender modify or just use, the natural value is wery stable and wont change unless
the oil is severly mis used- like overheating, extreme soot loading, extreme oci´s etc.
If the base oil doesent have a correct VI theres ways to fix it by using VI improvers, these have a tendency of behaving like a "Thickener"
when the oil gets hot..sounds good doesent it..well it usually doesent matter but these adds get worn out with use and
here comes the great debate about syn vs dino.
Very simplified, a 5W-40 syn usually doesent need any VI improvers since the base oil already is a 5W-40 but
the dino one needs to start with a baseoil that is say 5W-20 and improved with VI improvers so theres not as much robustness and wear tolerancens in the dino as the syn.
Note however that oil chemistry has gone a long way and theres a diminising line between dino´s and syns with some very high performing group 3
oils like most hdeo´s wich are severly hydro cracked dino basestock- one could argue if these are dino´s or syns.
Conclusion...choose the lower number for your lowest temperature and the higher by the book or like me a grade higher (30 -40).
I use a good hdeo 10W-40 in eveything due to the fact that i get it in 25 liter cans for a very good price compared to passcar oil.
Yes i know now every one will yell at me because of the mds in hemis...well these engines are not normal, the exception of the rule..or?