That's not true according to my 2005 Lexus GX470 Owner's Manual under Fuel Selection section, 91 Octane gas is NOT REQUIRED, but "Recommended for improve performance"
" Fuel selection:
Select Octane Rating 87 or higher. For improved vehicle performance, the use of premium unleaded gasoline with an Octane Rating of 91 or higher is recommended."
I've been using 87, 89, and 91 Octane gas and have not noticed any difference in MPG.