I find that the thicker is always better logic is broken. Thiner oils flow better this has a number of pluses like keeping the motor cooler, oh and yes better MPG.
Now why why wasn't 5-20 or even 0-20 not spec'ed for his car when built? Do any of you recall finding 5-20 on the shelfs at your local AZ, AA or Wally world in the mid 90's? I don't. But since the then Honda and Ford has seen the light.
Oil base stocks has improved.
Manufacturing tolerances become tighter.
Thinner flows better finding every ball barring and sliding part.
Better flow helps move in your engine faster, helping removing heat, and yes you do get better MPG.
Todays oils are much better today then before. 5-20 or 0-20 are great look for an oil that passes Fords Ball bearing wear test (Honda might have a test now too). Stick to blends or better as Ford's MotorCraft 5-20 is a blend. I used MC 5-20 in the summer and M1 0-20 in the winter in my Focus.