After reading through all 10 pages of this thread, and being thoroughly entertained along the way, I can't help but toss my observations in.
I have a 2002 ZO6 Corvette that I've owned since 2004 and have put over 160,000 km on. It currently has over 200,000 km on the odometer. I know this car. For those who don't, it's a 5.7L pushrod V8 with a manual transmission. The car has the original clutch in it but has had tons of "smiles" put on and has gone as quick as 11.86 @ 118 MPH before any engine mods were done.
The factory fill was M1 5W-30, which I continued to use for several years after owning it. Not being an oil expert I decided to follow the recommendations of a few folks who raced their Vettes and went with M1 10W-30 instead. If there were any operational differences between the two oils they weren't enough to notice. Both oils seemed to have similar hot idle pressures and that's the most attention I paid to the switch.
Two years ago I installed a new camshaft, lifters and high flow (+18%) Melling oil pump. Being a VERY high lift cam (.631/.644) the valvetrain is VERY noisy. I knew this going in. On cold starts I had pretty severe lifter tapping for 1-2 minutes and generally had to wait for coolant temps to get over 100F before setting off.
Since reading A PILE of information and consulting with a few folks here that seem to know their stuff, I decided to drop the M1 10W-30 and replace it with straight TGMO 0W-20. The Vette has both an oil pressure gauge and an oil temperature readout so I figured as long as I kept an eye on things I'd be OK and if anything looked off I could just dump it and go with something heavier.
I pulled into the local Jiffy Lube with the car nice and hot, dropped the 10W-30 M1 and dumped in the TGMO 0W-20. As soon as the engine built oil pressure with the new fill I could tell things were looking good... my clackety-clap valvetrain was now significantly quieter, more of a swishing sound. Idle was smoother with little shaking. the Jiffy guys were all standing around and noticed the same thing... they were all curious with what I was doing.
The next day on a cold start (oil at ~60F) the car started and idled so smoothly I couldn't believe it. This is a high lift cam but is not radical from a lobe separation or duration standpoint (Lingenfelter GT11), still on a cold start it would "bark" to life and chop-chop-chop, sounding pretty tough and quite loud. Now five seconds after starting it settles into a nice smooth low RPM idle, much quieter than before and with none of the drama. I can't wait to compare it to my friend's '03 ZO6 which has a stock cam in it. I bet it idles almost as smoothly/quietly. My cold start lifter tap is almost gone completely and I feel as though the car is ready to drive as soon as it's running.
With the car warmed up and the oil is at normal operating temp, between 210-220F, the idle pressure was around 29 psi which is about 3-4 psi less than it was with the heavier M1 30wt oils. If I deliberately heated the oil to 240F the idle pressure dropped to about 22 psi. This was with 6L of TGMO and the car needs 7L to be full, so I dumped in 1L of M1 0W-40 SM to bring up the hot oil pressure slightly (a light version of the CATERHAM blend). Now with oil temps of 240F my idle pressure is about 25 psi. Based on oil blending calculators and assuming the TGMO 0W-20 has a HTHSV of 2.6, my 6:1 blend should have a HTHSV of about 2.72 with a VI of about 212, cst@100C 9.2 and cst@40C 41.7.
Driving the car is MUCH improved. The light throttle responsiveness of the engine has improved so much it's like it's a different car. The effect is similar to having installed a lighter weight flywheel with none of the drawbacks... it revs so much quicker off idle, and rev-matching downshifts happens much crisper than I ever remember.
This is a big pushrod V8 with a big camshaft, probably the farthest thing from what Toyota developed this oil on. Rough calculations indicate the oil pump is drawing less than 0.5 hp to pump oil at operating temp at engine speeds up to about 2500 RPM, which on an engine that makes comfortably over 400 hp is rather insignificant. Yet the engine feels as though a huge weight has been lifted from it (pun intended). High RPM at WOT there's little if any difference from before... the butt dyno is not calibrated finely enough to detect those things, but at lower RPM there's a BIG difference in how smooth and responsive the engine is.
In my humble opinion I think there's a lot more to this oil than just the lower viscosity. Once again I'm comparing this to M1 5W and 10W-30 oils, not other light weight oils. Would I be able to tell a difference between this oil and another 0W-20? I don't know. In this big brutish engine, one that I would have thought less sensitive to small changes like this compared to typically sized engines, I'm literally blown away by the improvement a simple oil "change" has made. Is it the tri-whatever moly? Is it the ultra low viscosity? Is it the unicorn pee additive package? I don't know... I'm FAR from an expert on engine oil. All I can tell you is, fairy breast and angel hair analogies notwithstanding, I'm not putting anything else in this engine for the foreseeable future.
I support those who require "data" as evidence of a change and I salute you. I am that way well. All I can say as a conclusion here is:
Oil is cheap, don't knock it until you've tried it.