Can decreasing the oil viscosity decrease the gas mileage?

[barrys]

I've been running 10W40 in my Ranger for the 7 years I've owned it for no other reason except that that's what I used to put in my '63 F100 (which I really miss BTW). I just did an oil change and switched to the recommended 10W30 after reading up and talking to my mechanic.

I think I'm seeing a decrease in gas mileage with the lower viscosity/I] oil. Is that possible?

Just curious what other opinions people might have on this. I live in the San Jose, CA area where the climate and its range are pretty mild.

 

[Bajabronco]

It shouldn't make a difference, especially going lighter. If your mileage actually does drop I would put my money on it being something besides the oil.

 

[Big Jim M]

Just about anything is POSSIBLE! But in your case I'm thinking there would be no change in milage or in any case any milage change would be neglegable to the driver/owner.
Big JIm