Hello from France !

[Kara]

Don't remember exactly, but there is a table of different viscosity depending on external temperature, so basically you can choose between pretty much any viscosity, at least in South of France weather (usually from -5°C to +35°C).
It is also supposed to respect a specific Mercedes-Benz norm. I think it is more commercial than technical, but I found one with the MB norm, not more expensive than the others.
I chose the cheapest 100% synthesis from a big brand ; it was 5W30.
Also, these very small and quite high-revving engines are not specially known for their durability, they tend to wear. And there is no oil thermometer but it seems that oil runs quite hot. Si I did not want something too light.

Is there a risk that a too thin oil would be too liquid to be efficient ? And lead to premature engine wear ?

[Kara]

Back to the increase of mpg, one more change occured with the Smart : the A/C broke down, so not using it increases mpg !
But as it's a convertible I didn't use it that much.