I used white lithium grease and it totally eliminated the squeak with (~ 3 months so far) no bad side effects. I sprayed it on a piece of string and then used it like dental floss between the moving and stationary part of the folding axis. Back story: after I broke the plastic pieces of my driver's mirror (purely cosmetic) backing it out of the garage, I replaced the entire unit with one that I bought on ebay - the ebay mirror was fully functional and cosmetically perfect but it squeaked.
I was worried that silicon spray could migrate and harm the paint.