We live in Phoenix and she treats the gas pedal like an on/off switch 100% of the time. Maybe film strength isn't a product of viscosity and lower viscosity flows more volume.
Then again maybe film strength is a product of viscosity.
Let's ask this question. For trucks that operate under a certain load one weight oil is rec. BUT when operating with a heavier load another oil is specified - same engine, same truck.
NOTE: For 2500/3500 trucks with a 5.7L engine operating
under a gross combined weight rating of 14,000 lbs
(6 350 kg) or greater, SAE 5W-30 meeting FCA Material
Standard MS-6395 engine oil is recommended for all
operating temperatures.
It sure does seem as if the engineers do not think the light oil is up to heavy duty tasks. That is basically a 2500 truck with a 7000# load.
NOTE 2500 and up trucks don't need to meet CAFE requirements.
I seriously doubt that in a fixed pump with fixed orifices that there is a significant volume difference between oils of with such a small viscosity difference.
It takes a bit more power to pump a thicker viscosity but a oil pump such as used in motors is going to output a certain volume per revolution - unless the pressure relief is dumping the oil.