there are 2 points
fuel economy and oil consumption
regarding oil consumption, it depends upon engine oil base oil volatility, like 0w is more volatile than 5w, 5w more than 10w, 10w more than 20w,
hence chances are, there will be more consumption with 0w as compared to 5w and so on
like if you check Noack volatility figures, 0w oils are in 8-10% and 5w oils are in the 6-8% range usually (g/100g) (depends on the brand too) (if you're curious, it's according to ASTM D5800, you can read further on the internet)
it's one of the reasons why our desi ustads usually shift to thicker grades for reducing consumption, although that's not good for the engine if that thicker grade is not recommended by manufacturer, I don't recommend doing this. rather top-up oil whenever required
regarding fuel economy, 2nd figure (20, 30, 40, 50) matters there, because that's the operating viscosity, usually thinner grade delivers better fuel economy because there is less internal resistance and lower friction, for a layman, if I ask you to swim in water vs in honey, water= 0w and honey = 20w, it will take a lot more effort in honey, hence lower fuel economy, same for engines, lower fuel economy with thicker grades, better economy with thinner grades, for further on fuel economy, watch this clip
