I have a hard time believing that lighter oil can have a significant effect, or maybe even a measureable effect at all, on FE. However, there are some pretty convincing anecdotes in the first thread. Specifically, this one:
Normally, I'd have to say that this is placebo effect and is a total load of horse droppings. But too many Tundra owners have noticed a 3 to 5 MPG drop after an oil change. They then get out the receipts and/or go back to the oil change place and every time, it's traced to the dopey oil change place sticking in bulk 5W-30 instead of 5W-20 as clearly stated on the stupid fill cap. When they change back to the 5W-20, the fuel economy goes back up the 3 to 5 MPG.
Lighter weight oil was originally formulated and used by auto manufactures for fuel effecincy. I still use petoleum base 10w40 in all my cars. I think the heavier oil should give better wear protection. This is my opinion, but as they say opinions are like *******s everybody has one and they all stink. I don't think I'll be trying anything else in the near future since I have about 25 cases on hand.
I could have sworn that going from 10w-30 to 0w-30 in the tracker made a difference i could feel when cold, but i attributed it to placebo effect or something even tho I wasn't looking for an increase. Just wanted easier cold starting.
Going to an excessively heavy oil can cause a reduction in lubrication. The thick oil won't do as well in the same tight clearances that a light oil will. The caspian groove is designed for a certain level of oil weight as well and if you get heavier oil to the bearing, it has to rely on hydrostatic pressure(pressure from the oil pump) all the time and isn't able to build much hydrodynamic pressure(pressure created by the actual rotation of the shaft itself) to keep the oil film intact. hydrodynamic pressure is what really gives you protection against wear, hydrostatic is just a supplement for times, such as idle, where the journal speed is too slow for hydrodynamic pressure to suffice. Eventually the bearing would wear so the thicker oil would be okay but that shouldn't be a goal.