By spec’ing 5w-20, one can kill 2 birds with 1 stone. Better CAFÉ fuel economy, and you steer your customers towards high quality oils. The 5w-20 GF-4 is a known quantity as far as quality is concerned because the spec’s are so tough to meet for that class of oils. The high quality of these 5w-20’s is likely the reason they’re back spec’d too. An off the shelf 5w-20 is getting close, or closer to synthetic quality. Has
some wear been given up for fuel mileage -- probably, but in the overall scope of things it may not amount to much.
Is a 5w-20 the best for engine protection -- not likely. But it’s a reasonably good solution from the manufacturers POV.
If you are an engineer at GM, and the OLM is calibrated and tested using Grp II based oils, then you want your customers to steer clear of any possible Grp I based oils. In the late 90’s there were probably a lot of Grp I based oils hanging around in the heavier PCMO’s while the 5/10w-30’s were virtually all Grp II. So you put a warning in the owners manual DO NOT USE 10w-40 and 20W-50 oils -- not because of viscosity concerns, but oil quality concerns.
A higher viscosity oil has higher flash points, less volatility, and should be more thermally stable and provide a thicker oil film. So a 10w-30 using higher viscosity base oils should hold up better than a 5w-20 using the same class of oils (as someone already alluded too) -- but in the case of a 5w-20, one can use higher quality basestocks to make up the difference. So what’s better? Hard to say.
I think these viscosity concerns are way overblown. There should be no problem whatsoever running even a 10w-40 in an auto that spec’s 5w-20. That owner manual posted basically shows that anywhere from 5w-20 to 20w-50 can be used in an engine without issue.
We frequently hit -30C in the winter here. The viscosity of my 5w-30 on a cold start is ~ 5200 cP !! It takes 25min for the car to get up to operating temperature and in that time the engine is running with oil that has an incredibly high viscosity. Even when reaching operating temperature, the oil is likely running 40-60F cooler than someone who is driving around in Nevada where its 90F out. A 5w-20 is probably like a 40 - 60 weight in these temperatures -- and engines run fine. They just get worse gas mileage.
You get some clown driving through Nevada when it’s 105F out, 8000 miles into an OCI, 1 quart down, towing a 2500lb U-Haul at 85mph, using a 10w-40 Grp I that he got from the local supermarket in a plain black jug that said MOTOR OIL 10w-40 on the front
, and he is just asking for a sludging or oil related problem. With a Grp II, the chances are significantly less of having a problem because they’re so far superior. A Grp II based 10w-40 would be an excellent oil -- but how do you make sure it’s a Grp II when the 10w-40 spec’s can be met using Grp I based oils?
. To each his own. There are no solid facts just assumptions based on bits and pieces of information. Still, gotta love a good discussion right? 
