Since your Warrenty is till 2010 and Toyota STATES in the TSB not to use, your riskquote:
I would think that the 05 Corolla's can use 5W20.
I would not take that chance..
Toyota DID change the specs on the 2006 Corolla.
It does hold more oil. We know the dip stick has changed but we know nothing else. New windage tray? Different bottom end?
My warrenty is going to be up by the end of this year (if not summer) but I'm still going to stay with 5w-30. The UOAs and everything (incl excellent MPG) is showing me that 5w-30 is excellent.
Take care, bill
PS: If your dealer says it's ok. GET IT ON PAPER!