You can run 87 but to get maximum performance and power you will need to run 91. If you run 87 the computer in your truck will retard detonation so there is no engine knock and damage. You lose a bit of power running 87 but it won't hurt your engine.
I have run reg and premium in my 05 and I can not tell the difference at all. MPG is also the same, so why spend the money.
The engine is setup to run on reg. There is more chemical energy in reg. You will have to burn more prem to make the same power.
You will find that just about all the owner's manual say the same thing for all vehicles. They state a min octane rating and then there is a clause about addition performance from preminum fuel. In most cases it is all crap and it is written by lawyers and not engineers.
I understand that 87 has more chemical energy than say 93. But I thought that the 4.0 was a high compression engine, requiring the more stable premium fuel. And since running low octane in a high compression engine can cause pre-detonation, the knock sensors would pick it up and the computer would retard timing to avoid damage.
But I guess if compression were high enough to require 93, the ecu couldn't retard timing enough to avoid pre-ignition on octane as low as 87??
There are enviormental factors that determine wether or not it will detonate on lower octane, such as temperature, humidity, alititude. The engine will be less likely to ping in a cooler, more humid enviorment. I have learned my lesson in the past, go with what the engineers say you should use.