FYI
My wife's 09 3.6 with 2,963 miles I changed oil to a 0W-30 the first tank the car MPG increased from 25+ to 28.1 MPG. Her commute to work is 35 miles per day one way mostly open road @ 70 MPH.
We record every drop of gas, station and service to track fuel mileage.

Did anyone else have a jump in MPG around 3K?

Ken

My MPG was pretty much steady since the first day I got it.

http://www.gassavers.org/garage/viewgaslog/2504?

^^Gas log since my first fill-up, nothing too noticeable.

I'd recommend signing up for a website like "Gassavers.org" because you can easily track all the miles on your car and most importantly the mileage you are getting per gallon. Hope this helps!

Mine's been pretty consistant as well. Too many variables to determine why your mileage went up so much.

I have been using a simple excel spreadsheet on my cars/truck to track MPG, Preventive Maint history etc.
I track brand of gas and location of each fill up. I will follow up on post after another month of driving estimate additional 2K+ miles.

Ken1

you used 0W-30? I thought most use 5W-30? am i missing something?

I have used 0W-30 oil for the last 12+ years in V-8/V-6 and one lawn mower without any problems. The wife's last two cars 97 Monte Carlo w3.1 and 04 Malibu w3.5 all had over 100K with this oil.
The weight of oil causes less friction and engine runs cooler = more HP and MPG

Ken1

I have used 0W-30 oil for the last 12+ years in V-8/V-6 and one lawn mower without any problems. The wife's last two cars 97 Monte Carlo w3.1 and 04 Malibu w3.5 all had over 100K with this oil.
The weight of oil causes less friction and engine runs cooler = more HP and MPG

Ken1

Interesting. Thanks for the info! :)

you used 0W-30? I thought most use 5W-30? am i missing something?

I'll stick to what GM recommends 5W-30W.

Isn't 0W-30 just 0W when cold, and when engine is warm acts as a 30W? so therefore should be "theoretically" the same or very close to a 5W-30? (ie only differences are while engine is warming up which is pretty quickly I've noticed). Just curious....

0W does nothing but provide MORE protection when the engine is cold. Cold doesnt mean -40 below but 90-100 degrees. Remember the car operates at 210-220.

Also, most harm is done at what??? Start up. Why not have more protection at the most venerable time for the engine.