Originally Posted by Andrews-bad-SE
... What kind of mileage are you getting with the 0w40? I work far enough from home to worry about gas mileage now. I was actually thinking of switching over to the Mobil 1 0w20, as it states that it meets the same cold-start and hi-temp lubricity standards as the 5w20. Is the 40weight not a little thicker? ...
I can't say what kind of mileage the Focus is getting (my daughter drives it), but I can tell you that I see no difference in mileage on my Saabs changing from one Mobil 1 syn to another. I've used Mobil 1 10W-30, 0W-40, 5W-40, 10W-40 & 15W-50, & they all return the same fuel economy.
I saw a noticeable (10%) benefit switching from conventional oil to Mobil 1 in my Saabs, but nothing really changes from switching between different Mobil 1 viscosities. I think the car manufacturers notice something like 1% fuel efficiency increases by changing to lighter viscosity oils, but that's under controlled test conditions. Once you & I are driving our cars in the real world, it's harder (impossible?) to see any economy benefits from viscosity changes alone.
Now, if I had a very efficient (60+ mpg) hybrid, then I might be able to tell a difference simply because my entire vehicle is already optimized for fuel efficiency. So changing one thing like engine oil viscosity has a larger impact on the whole 'efficiency system'. But not on my 30-35 mpg cars.
You'll see bigger fuel efficiency gains by keeping on top of routine maintenance & modifying your driving habits. For example, I see a 30%-35% fuel efficiency increase by driving 65 mph on the highway instead of 80 mph. I used to have an hour-long commute which had about 30 miles of highway driving in the middle of it. Slowing down to 65 mph added 5 minutes to my total commute time. I didn't care; for a 30% efficiency gain, it was worth it to me.