Mobil1 0W-30 at Walmart in 5 qt containers
I've seen some earlier posts on where to get Mobil1 0W-40 in larger (cheaper) containers. I haven't found a source other than Autozone in 1 qt bottles for about US$6/each.
Today, for the first time, I found some Mobil1 (full synthetic) 0W-30 in Walmart in the 5 quart container for $23. It has a green top where all the others have the silver top. They're pushing it as environmentally friendly since the lower viscosity could improve gas mileage.
I suspect that unless you live where it is very hot or you run your vehicle real hard, the 30W will work as well as 40W. Of course, you could always top off the last two quarts using 5W-40. Mixing doesn't hurt - they blend together.