Daaaaamn, so many great replies here! Thanks fellas!
A few thoughts. Re
@rob1715 "if WS is so bad, why do so many 'yotas last so long running WS?" This is a good point and definitely something I considered. I'm inclined to think it comes down to the mechanical design and implementation far more than a marginal improvement in the type of ATF (which is what I'm going for). Toyota seems to be amazing at building the best, most-reliable vehicles in the industry....that's their specialty and nobody is better at this. When it comes to oils/fluids, clearly they do an excellent job here too, but it's not their area of specialty....they're not the best of the best in the world when it comes to making oils. I believe that title goes to Amsoil. I think Toyota makes a cost/value calculation. They buy their oil (or at least the base) from a third party, who has to mark it up, and they need room left for their own markup to dealerships and then a massive markup to the end consumer. If they used super high-end oil with tons of $ in R&D as their base (such as Amsoil), there'd be no margin left. So they make a typical engineering efficiency calculation like all car manufacturers make for every part in every step of the design process that the component in question (in this case, WS oil) is good enough, maybe even very good, while being cheap enough, maybe even very cheap. That's what it seems like to me anyway, as a random internet person with absolutely no understanding of the actual way Toyota Corp works or what criteria they use to make their cost/value calculations
Basically, is Toyota WS good, maybe even very good? Good enough to keep many already very, very well built, well-engineered vehicles going for many thousands of miles? Yes. Are many of these vehicles *cough*Hilux*cough* so good that they could probably run on Canola oil? Yes. Could WS be better? Yeah, I think it probably could? Have there been examples of Toyotas surviving EXTREME torture tests, such as running them with no oil, or on original oil after 100k miles or other such ridiculous things? Yes. So I think that makes the case that why Toyotas last so long is mostly due to mechanical variables, not so much outstanding OEM oils. I think there's room for marginal (not huge) improvement there.
Amsoil is to oil design and manufacture what Toyota is to vehicle design and manufacture. I'm just trying to get the best of both worlds.
Re
@Mainia I've heard this kind of thing from several folks in online forums or on reviews (ie. that original OEM fluid or some cheaper fluid was trashed, new Amsoil or other such fluid looked brand new at the next change after 30-50k miles). I notice this is often on newer vehicles after the first fluid change. I'm no transmissions specialist, but could it be that new transmissions have to be worn in? I know this is the case with difs and transfer cases. The fresh gears have to shear off a bit of fresh sharp edges and milling marks and just generally settle into their place until they're relatively smooth. I would tend to think the first oil change will be one of the worst, but I could be wrong.
From my generally reading of replies, I'm thinking it's gunna be either Idemitsu or Amsoil SS. Re
@FowVay do you really think the slightly thicker Amsoil ATL is significant? Amsoil has a significantly lower pour point, so it seems like the extra viscosity won't be a problem in the cold(???). Amsoil KV @ 40 = 30.8, Toyota WS = 23.1....Amsoil KV @ 100 = 6.3, Toyota WS = 5.3. I'd be curious to see the curve on these. It looks like the Amsoil is more similar to WS at higher temps, starts thickening up at mid-temps (40C), yet retains fluidity at super low temps (WAY lower pour point than WS)....yet it somehow ends up with a lower VI? I'm still not sure how that works. Seems like if it's similar at high temps, but remains fluid much lower, the VI should be higher (???) than one that's similar in high temps, but doesn't go as low? I'm still confused on this.
Anyway, thanks everyone for all the input and great info!