I just bought a 2010 Toyota Prius and it requires 0W20, 5K OCI's. While perusing a Prius board I came across this post by Toyota marketing put up yesterday. I thought you guys would be interested in their comments on the oil.
We know there's been much discussion over the new synthetic oil guidelines from Toyota. As was mentioned back in April, the new oil will eventually roll out across more lines and manufacturers and provide lower emissions and improved fuel efficiency. The owner's manual lists the use of 0W-20 and 5K oil change intervals so we can't really comment further on what would void your warranty. The synthetic oil should now be available at all dealers and should very soon be available in bulk, which will reduce the costs for everyone. Genuine Toyota Motor Oil 0W-20 contains a Toyota specific additive package within it, making it not interchangeable with other synthetic oils of the same weight such as Mobil 1. We can't say much more now, but stay tuned for more info shortly, our experts are doing their best to provide more information as it becomes available. Best, Erica G, Prius Product Marketing, TMS, USA