Toyota Repair: 1998 landcruiser & premium fuel, premium fuel, fuel economy

Question
My owners manual says "91 octane or higher fuel required".Does this vehicle have to run on 91 and what will happen if I run on 87 ocane.With the cost of gas as high as is,it would save me a bundle on gas.Why does it say 91 is required?

Answer
Nothing will happen, I use 87 in my '07 FJ with no problems, it's been said that there may be a slight fuel economy increase by using higher octane which would offset the cost difference, you can do some comparison MPG tests if you like, I have not done any so I really do't know if that's true, I'm getting about 15 mpg city and 19 mpg highway using 87 octane.