Hi fpracha,
I get asked that a lot, but fuel economy isn't my reason at all; it's a nice consiquence though.
The reason for wanting the lightest oil possible on start-up is partly philosophical and partly practical.
Firstly all motor oil, even the 216 VI Toyota 0W-20, is heavier than is ideal on start-up even if it's 120F outside. You still can't start an engine cold and use even modestly high rev's without the oil pump going into by-pass mode; it's just too thick until you can get some heat into the oil to further thin it out and that takes time which can be frustrating if you are anxious to extract full power from your engine.
Running a high VI oil is like having a head start-up on the warming up process vs it's low VI equivalent. The advantage in cold driveability is immediately apparent. Due to the lower oil drag the engine behaves as if it's already partially warmed up; rev's easily and in the case of my Caterham which doesn't have electronic idle control will idle immediately upon starting without any choke necessary.
It's unfortunate that most BITOGers don't have oil pressures gauges in their vehicles as they are effectively blind to the lower oil pressure on start-up that results with a high VI oil.
As a consiquence an oil's VI is just a number that doesn't seem to have much practical importance except for very cold starting.
When I say the Toyota 0W-20 is 35% lighter on start-up compared to a typical 5W-20 at room temperature that doesn't impress most, but if they had an OP gauge in their car to confirm it and they knew that they normally had to drive around for at least 10 minutes to heat up the old 5W-20 oil to realize the same OP, that would make more of an impression. Then the more technically inclined would realize just how great an ultra high VI oil really is in the daily running of their vehicles.