You could have done what I did. I went to Walmart, bought the 0W-20 out of my own pocket, took it to the dealer and had him put it in. It does make a difference in MPG. I would never let him put in 5w-20 even if it was the alternate recommended unless your in a very high heat area.

I feel you would see a huge difference in MPG between 0w-20 and 10W-30. You can get great gas mileage by using 0w-20 or you can get a long lasting engine by using 10w-40. Since a lot of warranties only cover drivetrain to 36,000 or 50,000 miles most manufacturers are recommending lighter weight oil to meet EPA and gov requirements for FE and the heck with engine durability. I think we may start seeing many engines expire before they reach the 10 year mark.

My dealership has used 5W30 on both my oil changes; if this has made any difference to my fuel economy, the effect has been tiny compared to the differences that temperature and driving pattern make.

What are you getting in MPG now and what part of the country do you live? I am in the Great Lakes of PA and getting just under 40 mpg in the city with 0w-20. If you live in a very hot area with consistent tempts well into the 90's, I can understand the heavier weight oil.

Boston area, got 37+ on my last tank and probably 40 so far on this one (computer says 43.5mpg, after about 200 miles). In winter, more like 32mpg.

I'm not an automotive engineer but I seem to remember that a using heavier oil than recommended can lead to premature engine wear also. If the oil doesn't flow fast enough and/or the bearing clearances are small enough that the oil doesn't reach them properly then engine wear could be greater with a heavier oil.

Actually, the worst oil weight to run is one that covers a large span, such as 10W-40. The reason being is that in order to span that viscosity (in conventional oil), you need to add polymers to the oil. These break down in an oil first.

I would not be overly concerned about running a 0W-20 oil in a synthetic. Synthetics have virtually no polymers. The base product can span the temperatures required without the use of polymers. You might even find that a 5W-20 synthetic and a 0W-20 synthetic are the same oil, they are just so good, they passed passed two different tests using the exact same oil.

It gets late early out there. - Yogi

Which is also how they make processors... They come out of the same wafer and get tested. If it works at 2.4 Ghz, the processor gets a 2.4 Ghz sticker... if it works at 2.6, it gets a 2.6Ghz stickers... same processor, just came out better while being manufactured.

Oils is the same way and that is why you'll almost never find a difference in price between a 0w-20 and a 5w-20 (wish I could say that about processors!)