I had an 05 GX and ran 87 octane with no problem.  I now have an 06 GX and I
noticed that Lexus has installed a decal inside the gas cap door which calls
for Premium Unleaded. I've tried all 3 octane grades and haven't noticed a
difference. I read that the engine will cut the timing slightly and using
the 87 octane fuel will not harm the engine. It will just give a slight
decrease in performance. But of course most folks will say if you can afford
a Lexus, you can afford to put the proper fuel in it.

My dealer, and the book, told me to use Premium in my 04 RX330. A couple of
months later while in for an oil change I asked my salesman. He told me he
has the same engine and uses 87 octane with no problem.

Confused, I ran my own test. I ran about five tanks of 87 octane, then five
tanks of premium, doing essentially the same type of driving. My results
showed about a 1.25 to 1.50 increase in mileage using premium. I suspect
most would see the same difference.

Bottom line; dollar wise it is about the same. Use the recommended premium.