We just finished our second trip in which I changed oil to check my MPG. We used the factory fill 5W-20 for the first trip and our MPG was 36.3. I changed to 10W-30 for the second trip and we got 36.4 MPG. Both trips (at a little over 400 miles for each) were over the same Interstate and I set the cruise control at 70 MPH. The temperature present during the second trip was about 10+ degrees hotter with the air conditioning being used for the distance.

While the experience wasn't 100 % laboratory accurate, it did indicate to me that I will not be suffering a mileage penalty for using the 10W-30 oil. We will be taking a trip over 2,000 miles in the near future with more traveling weight at the same 70 MPH speed, but I expect I will see the 36 MPG to continue. So far, we are happy with our MPG for our new Accord. And it has excellent acceleration for a 2.4L engine.

 

the difference between the 5 and the ten is the viscosity when cold, or when the oil has been sitting for about 4 hours. the 30 indicates how viscous the oil will be when hot, so when the oil is hot, both are the same. 5-30 is usually used in colder climates because it is less viscous when cold, which helps cold starting lubrication. in hot climates, 10-30 is used.