ATF WS or full synthetic?

It is ALWAYS better and best to run what Toyota says to run in their trasmissions. Always.
The cost difference for anything else is pennies.
The Prius transaxel/mission is electric. Putting the incorrect fluid in it could ruin the generators.

 

Yes in Corollas and non-hybrids and Yaris and all that You can run what you want but in the electric business you want to make sure the stuff meets the Toyota standard for that whatever that is won't smoke your motors or your generators Just about anything will lube the CVT it's the motor generators that are the problem here.

 

Stick with real WS. If you had to change it every month it would be worth finding a substitute, but this is the opposite case. Even changing it once could be considered over-maintenance... just not worth screwing around with alternate products.

 

Toyota says flat-out, to use only Toyota ATF WS, and that anything else may cause damage.

 

Yes it's Toyota should say to keep selling their product No problem Good marketing I would hope now that with the myriad of hybrids out there that other companies would have fluid that would be up to snuff for this duty but there again it is possible that nothing is happening but I seriously kind of doubt it there is some standard that you have to meet for the electric motor or the dielectric type fluid or whatever we're going to call it or whatever they call it there has to be a automotive standard for that and as long as your fluid meets that then you're good to go that's the same thing with all the oil hub of years ago that went on for 30 years and people argued about oil and it was just crazy. But like it's been said above as this is never going to happen usually people never change his stuff or they might get to it once and after that the car is no longer and out of sight and out of mind usually not long after they change the transmission fluid probably because the mileage and all that is right there where things start to happen and then the smart person realizes this is not the smart thing to be doing and that's done.