Why does my 2011 Toyota Sienna V6--FWD Manual say to use 3 different octanes? Which one do I use?

Whenever I ask someone what octane I should use in my 2011 Toyota Sienna, they say..".Just put in what the 2011 sienna manual says to put in!" .....well here is EXACTLY what my 2011 Sienna manual SAYS to put in...Exact Quote from manual...."FUEL TYPE: Use unleaded gasoline (Octane rating 87 [Research Octane Number 91], or Higher. " End quote. So...whats with that? If Toyota don't know what octane I should use in the new Sienna, how am I to know? And why would Toyota even recommend a high test gas if Sienna does not need it? The dealer told me to use 87, then they filled it with 89, and the manual says you could use all three!!!! Some web sites I have seen on the Sienna state 91 should be used exclusively because it is a high compression engine...others say use what ever you want, and Toyota won't commit to any single octane at all! This van has to last me for many years. What octane gas does a V6 2011 Sienna need???