I agree with the theory of everything you said and I don't like "all purpose" this and that but I have never heard of a case where a car manufacture denied a warranty because an non OEM fluid was used. if you put ATF fluid in the Engine....sure, if you used 0w-40 , maybe but because you used Mobil 0w-20 Engine oil.....I would be shocked
I don't think encouraging people to use $2.5/qt oil from behind John's barn is a good idea but I don't think encouraging only use Toyota Fluids to maintain a warranty makes any more sense. (not saying you are doing either, but this thread is headed that way)
VW denied warranty of many engines in the past because owners did not use the specified oil (VW oils are specified not by weight, but by a VW specific code such as "504 00" based on internal requirements for oil performance), people erroneously thought "any 5w-40 or 5w-30" oil should be good enough; they failed to realize that a VW specific oil is engineered specifically for the engine with additive packages and chemical make-ups that are different between spec. A 504 00 oil and a 507 00 might both be a 5w-30 or 5w-40 on the bottle (VW has a 20+ page TSB that shows all current manufacturers of oil, their offerings, and what spec they meet; many of which range between 0w-30 to 5w-40 for the same spec but made by different manufacturers), but run a 507 00 (diesel oil) in a 504 00 (typical gasoline) car and you have issues, or vice versa.
VW got sued a while back because in North America for a while there was a specific VW spec (I forget which, I really need to find the specific case as it was an interesting read/outcome) that was only available from the dealer. People went the cheap route, ruined their engine and got mad when VW said "you used the wrong oil (they tested the oil, which from the few Mercedes and VW guys I have talked to is actually very common practice in engine failure warranty cases), you don't get warranty" they won the lawsuit because their excuse of "it's not our fault only one manufacturer, who happens to supply VW brand oil, is the only suitable oil that meets the spec in North America". Conversely Mercedes tried to pull the same thing and when investigated it was found most dealers didn't even know what they were putting in their vehicle (M229.3 vs. M229.31, ect.) and they lost because they themselves were not using the correct oil sometimes.
That's why GM says you need to use "Dexos" oil now in their engines, they learned from VW on that one (granted in reality Dexos is more of a marketing gimmick and almost all non-factory oils are easily Dexos oils). Toyota is one of the few who are very liberal on their oil spec, as long as it meets SAE Viscosity and API index ratings it's good to go, you'll probably never have warranty denied if you used say whatever 0w-20 oil you could find because it will easily meet the SAE and API ratings Toyota specifies; unlike some manufacturer's who are more militant.
Conversely you would have your warranty potentially denied if you used a non-Toyota approved coolant (considering out of my own personal experience Zerex Asian is the only one that matches up with SLLC, it's easy to do and often done by people) though realistically, the damage to your engine would become significant far out of the warranty period anyways; cavitation, metallurgical reactions, electrolysis ect. But Toyota could say, "you used a non-factory approved fluid that contributed to the damage your engine, Toyota is not liable for the damage because you used a fluid that does not meet the specifications." A lot of those "Universal" coolants say they meet a lot of specs, but often contradict themselves (i.e. I've seen one Autozone special that said it was good for both Dexcool AND SLLC, which is complete BS, but the bottle said it was compatible with modern Toyotas).
My point is, that while no in a Toyota you wouldn't get your engine warranty denied because you used Mobil 1 0w-20 vs. Genuine Toyota 0w-20 (which honestly I think is made by Mobil 1?), but you could have it denied in another brand and it has been done before (by the way, Mobil 1 5w-40 is okay for Mercedes engines as it meets their specifications for M229.3, but it does not meet VW's or BMW's specifications; that requires Mobil 1 "European Formula", I had to reiterate this constantly at my old dealership when doing used vehicles) because people would mistake the SAE rating to equate to oil specification, when in reality in some brands it's far more complicated than that.
Post Note-
This is mainly why I am saying that if you are going to use a non-factory approved fluid (i.e. any fluid that Toyota has not officially approved or licensed for use in it's vehicles) you better be sure it matches the specs the vehicle requires; not just blindly believe whatever BS they may say on the bottle. Because IF it ever came down to a warranty issue where they decided to deny it based on the use of an aftermarket fluid; you'd have to prove that the fluid meets the spec, and I doubt corporate lawyers are gonna let "the bottle says so" slide in court without serious evidence to prove it really is the same fluid.
Though I'll be honest, in my experience as a technician I haven't seen warranty be denied because the wrong fluid was used (then again, I haven't seen much non-factory fluid in vehicles still under warranty and even less fluid related failure; so we're all talking about a rather unlikely scenario here).
, use them hard and yet baby them. We want them to last us a very long time. Isn't the peace of mind in taking control of the entire choice of and change interval for lubricants worth the peace of mind itself? It is for me and figured many others here would feel the same and benefit from the key quote I shared. 
