I am amazed you are out of warranty already. I sure would not deliberately risk my warranty by putting in incorrect oil on purpose and publishing it on the internet. 0w-20 synthetic for at least the power train warranty.
I think this was a question. The owner's manual sums it up well, saying 0W-20 is "required" but that a higher viscosity might be more appropriate in certain circumstances (I paraphrase). Mobil1 0W-30 is an excellent "universal oil." Fuel economy might take a slight hit, though, vs 0W-20. The Prius repair manuals, linked to on PriusChat, indicate acceptable oil grades go all the way up to 20W-50 for the Prius. Let the arguments begin.
This was scanned from my 2010 Owners Manual pgs 560-561 and does not conflict with the repair manual. This allows you to choose based on temperature and load conditions within the 0W-20 to 20W-50 spec given. However 0W-20 is given for normal service. http://priuschat.com/forums/attachment.php?attachmentid=33673&stc=1&d=1323557778
0W30 may be a better choice for the engine, I think the only advantage to 0W20 is slightly better mileage. I agree the warranty does seem to give us permission to use heaver oil under some unspecified conditions. How about using 0W20 in the winter and 0W30 or 5W30 in the summer?
We may find out, there are rumors in one of the rough start forums about a TSB coming out to fix the problem. I have always thought the rough start issue was fuel/air mixture related myself.
