It would be cool if Ford or another manufacturer recommending 5W-20 for all its engines perhaps had a 30 minute special on TV showing why using 5W-20 is better than 5W-30 which most all of the auto manufacturers use to recommend. GM is still recommending 5W-30. Are they not concerned about CAFE standards and saving vehicle owner's .5% of their gas?
I am profoundly curious as to why Ford, Honda, Toyota, etc. decided to make engine clearances and bearings and such use tighter clearances so they would have to use thinner oils. It doesn't make any sense to me. In fact, I wonder if it is true.
However, quite a few of the 5.7L Hemi owners have to run 5W-20 as the engine shuts down cylinders when the vehicles using that engine are at a constant speed. The thing that makes this 'cylinder killer' operate does indeed have tight clearances.
Then the old argument that to me is still a very valid one is heat. You want the oil to be thick when the weather and engine are both hot. It really isn't an argument. It is a fact. Thick hot oil protects engines better than thin hot oil.
Remember, with 5W-30 or an other multi-weight oils, hot 30 weight is much thinner and more fluid than cold 5 weight in winter.
To be honest and no offense to anyone, I don't think any of us here can really answer the question. None of us are in the know.
But one thing is for certain, it is generally best to follow the recommendations in your vehicle's owner's manual.
My 1998 Ford Mustang GT with the 4.6L V-8 engine is one of the cars Ford says to now use 5W-20 in as opposed to using 5W-30 which the vehicle's owner's manual has always specified and the car has always used. I know the clearances and bearings in my engine are no tighter than when they were all new. I sent an e-mail to Ford which will hopefully be answered asking why should I use 5W-20 instead of 5W-30. If I receive a response, I will happily share it here on BITOG.
This image from a vehicle owner's manual (not sure of the model or make) interests me...
As you can clearly see, both 5W-20 and 5W-30 protect to the same temperature range and cover the same temperature range as 10W-30. All 3 of these weights appear to cover you to 110°F outside temperature. I am skeptical that a 20 weight hot oil can protect an engine as well as a hot 30 weight oil.
My state, Illinois, is a funny weather state. We have cold winters which sometimes get to -10°F and summers that get up to 105°F. Other states like Texas get hotter than Illinois. The point I am trying to make is that states with hot temperatures probably want to be using at least 5W-30 in the engines of vehicles there.
Anyway, I think we are all still learning. To me at least, it makes since that a lighter oil can lubricate better in that it can move more heat due to its thinness. But if the engine overheats which happens, can the film left by 5W-20 protect the engine? Here is a quote which really makes me wonder this...
http://www.machinerylubrication.com/article_detail.asp?articleid=518&relatedbookgroup=Maintenance
"In other words, an SAE 20 at 190°F is about the same kinematic viscosity as an SAE 30 at 220°F, which is about the same viscosity as an SAE 40 at 240°F. This approximation works well in the 190°F to 260°F temperature range. One might be surprised at the slight amount of difference between straight viscosity vs. multiviscosity oils with the same back number (for example, SAE 30, SAE 5W-30, and SAE 10W-30).
If an SAE 50 oil at 260°F is as thin as an SAE 20 oil at 190°F, imagine how thin the oil film becomes when you are using an SAE 5W-20 and your engine overheats. When an engine overheats, the oil film becomes dangerously thin and can rupture."