0w-16 weight oil is the next generation of oil aimed at reducing frictional losses within an engine due to the oil. To the average person it seems too thin to be safe, but they've been planning and testing this stuff for several years now and with very good results. Toyota isn't going to do anything stupid to harm their reputation of providing quality products.
Edit: Japan has been using 0w16 for almost 2 decades now...that's pretty tried and true. Not really sure why it took so long for the US to join the ball game.