According to a source I saw recently (don't remember where), USA citizens are visa-exempt for ninety days in Brazil, with a single renewal possible.
It is not clear from http://saofrancisco.itamaraty.gov.br/en-us/visas.xml which type of visa would be appropriate for a retired person to stay in Brazil longer than 180 days. (Non-working, income sufficient to live long term.)
Asking the San Francisco consulate, the response was the retirement visa which assumes intent to live there permanently. When I clarified my request, they said there "He/she cannot, on a visitor status."
This is also not clear. It appears that I would have to apply for permanent residence, and then just leave when I am ready. Another source says the residence permission is canceled when you do that. Would such a cancellation prevent future visits? Or is there an option that they aren't telling me? Would applying for permanent residence when you don't intend to be permanent be considered making a false statement?