So I had my 8,000km service on my Micra before the fall hit. I just bought this car in April and I already put 10,000km on it!

The stealership put synthetic 0W-20 in it, when the manual calls for conventional 5W-30. I know they probably put that oil in every one of their changes. I didn't pay for it because I got oil changes with the car.

I do notice my fuel economy has changed, average of 7.5L/100km up from 6.8L/100km with the factory oil in it.

Am I nuts or did the oil do that?

Should I be swapping back to 5W-30?