Independent studies out of Europe, compare vehicles in Canada, running 0w20 vs 5w30 in Europe.
1.2%-2% increase in fuel mileage 5w30-0w20 a win for 0w20.
20-40% increase in metal to metal contact, particle count and wear on visible engine parts on 0w20 vehicles vs 5w30. Both synthetic. 5w30 wins here by huge margin.
1.5-2% decrease in emissions on 0w20 vehicles due to increase fuel economy.
Some newer Toyotas are even using 0w8, even thinner then piss water....
Bottom line for the above, a 1-2% marginal increase in fuel economy and emissions for 20-40% less reliability....
Use 0w20 or even 0w30 in the winter and switch to 5w30 in the summer, your engine will love you.
Study is based upon a 22 Honda Civic SI
Runs 0w20 in Canada all year round
5w30 in Europe all year round