Difference in MPG using different oil viscosities?

[fantastic]

I'm

Yeah sure, that's it.

Huh?

 

[gfh77665]

I used to drive a 2004 Chevy Avalanche, its gone, so now I use my 2017 Ford Fiesta. That's 15mpg (on a good day) vs. 40mpg.

I don't even think about oil viscosity, LOL.