I do concur with above statements. However, I have seen 1 instance in my career where a sudden change to thin oil not specified for a certain motor caused damage (early 90's grand prix 3.1L).
That being said, if your oil pressure looks good and the engine sounds normal you're probably good to go. Just monitor oil consumption on that year motor, especially with thinner oil.
As far as increased mpg benefit, it's negligible at best. Maybe 0.2 - 0.3 mpg better, and usually just on warm up. If you're the person who jumps in at 20F and hits the road as soon as you have rpm, then you might save a little bit. If you are like any caring, educated driver, then the brief warm up in cold weather pretty much negates what the EPA was trying to accomplish in their 'conservation' efforts. LOL!
There's a reason why any work truck that has a similar architecture motor runs 5w-30 instead of 0w-20. Different C.A.F.E regulations.......and longevity