I wouldn't recommend straying outside of the Toyota recommended 5,000 mile oil change. As such, I think you are pretty safe if you stick with regular Mobil1. It can easily go past 5,000 miles, however there is no good economic reason to do this. I'm more inclined to want to keep my engine clean and lubricated rather than save a few bucks on motor oil.
Amsoil will only prove itself if I can go 10,000 miles between changes with no ill effects. Right now, it's rather stiff at $25 a gallon. The specs read well, though. 35,000 miles requires a few filter changes...
I use dino, since I change it every 5K like the warrantee requires. I don't drive in freezing weather much at all (SoCal), so I wouldn't expect a benefit from synth. 5K is overkill but I do it, along with a new filter, which is no doubt even more overkill. But I do it myself, so $10 total every 5K is cheap. I keep it clean and cool, and leave the rest up to the good Toyota engineers to make it last forever.
Royal Purple is good stuff. A full synthetic. It has very good viscosity properties and low and high temperatures also. If you can find it on the shelf in your favorite car parts store, it might cost a little less than Mobil1, which costs as much as it does because of its name and the reputation built up around it.
For a synthetic oil, it probably is true about Mobil1 being an excellent choice. From what I've seen, the extended milage Mobil1 costs more, and there isn't any reason to get it if you are going to follow Toyota's recommended maintenance.
I would also say that if you're going to ignore these requirements you could have trouble if you try to make a warranty claim. I have heard that certain other auto manufactures have denied warranty claims because of the owner using 10W-30 oil instead of 5W-20 as they required. It is just another point of contention if something goes wrong and there is a warranty claim. I really don't see using 0W-30 unless you live where the temperature regularly dips to other planet coldness of -40 or less, and if you do live there you're probably tough enough to twist their arm on any warranty claim. It is also my understanding that 0W-30 oil isn't just 10W-30 oil with a few more additives to get to the 0W. I think the characteristics of the 30 are also different.
From what I have been able to check, if you don't want to use the Mobil1 some other choices are Penzoil Platinum, which is what I've been thinking of for my Prius when it is time to change the oil, or if you really want to live it up you can use Red Line, but that's $8/qt and not readily available. Valvoline SynPower 5W-30 is supposed to be pretty good too. Amsoil may be great too, but I'm a bit hesitant becuase it seems to be pushed so much by their distributors, but maybe that's because they think it really is great. The only possible negative I have heard about the Mobil1 is that it is a "thin" 30, because it is toward the lower part of the range of acceptable characteristics to be called 30. But I am using the Mobil1 in my mom's Prius. In my Honda Rebel 250 motorcycle I've used Mobil1 20W-50 from 600 miles and it now has over 90,000 miles on it, but of course 20W-50 is much different from 5W-30.
Also, it seems that the oil filter is really much more critical than the oil itself, so that may be where we should be doing the questioning. For now it seems I have to stay with the Toyota filters, but I would like to find something better. Somewhere I heard about a different and larger Toyota filter which might be better, but I don't have the model number and have never used it.