No point in second guessing the car maker. CAFE or not,
I wouldn't second guess them.
Bottom line is stick to what is spec'd for the engine,
I think it is very appropriate to "second guess" the manufacturer. Just look at all the recalls and service bulletins out there, all the oil/sludge/wear issues with engines,and also power-train issues,etc.
The manufactures can no longer afford to do all the R&D like years ago,they are now going broke! Much of it is now done using computers/virtual programs,and the rest is done using the motorists as guinea pigs. The mentality is, build the car,sell and deliver it to the masses,and make corrections we missed in the lab....hence all the issues in the field with bulletins/recalls.
The 5w-20 motor oils is relatively a new thing and the automakers are still on a learning curve. My 2009 v6 mustang calls for 5w-30. My 2008 Jeep "recommends" 5w-20,but the Allpar site recommends 5w-30,stating the oil pump is really spec'd for 5w-30. Allpar also lists much shorter maintenance intervals across the board than what the owners manual states. The owners manual states for example,change the transfer case every 60-120K,while allpar states every 30k,and the MFG/TECH-expert for the unit itself...states every year or 15k because the unit is hard on the lube.....go figure!
The 5w-20 is usually a mineral oil with the use of of a cellulose oil filter. How could a mineral oil 5w-20 function better than a true synthetic 5w-30, or a 0w-30 for winter use using a premium synthetic/micro-glass oil filter? What does 5w-20/mineral oil do at 255 F+, verses 5w-30 100% synthetic regarding oil pressure in critical areas of the engine.
Remember........True 100% synthetic oil is a better "heat transfer agent" than mineral oil,meaning it cools better! I recall M1 "claimed" years ago(100% PAO) that their 15w-50 flowed at least as well as a 5w-30 mineral,but protected better than a mineral 20w-50.
Running a car on a Dyno is a great start,but real world conditions always prove to be much harder on the engine,and the oil. Repeated,Cold/dry starts in the frigid winter that cause negative tolerance/clearance in the piston to cylinder area,incomplete warm ups,condensation,gritty oil,bad fuel,etc,all have shown to be very hard on an engine/oil.
When I start seeing service bulletins that state........"do not use anything other than 5w-20"........only then will I listen,somewhat! I know of people who are using 0w-30 in the HEMI engine that "mandates" 5w-20,and the 0w-30 100% synthetic is working perfect in the Hemi engine,as just one example that even mandates are not totally accurate!
Synthetic article reads.....http://www.fordscorpio.co.uk/sythoil.htm
5w-20 vs 30.........quote....
"5W-20 is used PURELY for gas milage for Ford. The Ford Explorer calls for 5w-20 yet the Mountaineer that uses the EXACT same motor, all parts are interchangable and has the same engine code, calls for 5w-30. My S10 uses 5w-30 or 10w-30 I've gone to amsoil15W-40 desiel oil with out a problem. The reason for doing so was for higher oil pressure during cold days."