Wow, a lot of misinformation around here regarding the use of premium fuel.
The main 2 factors that contribure to using 91+octane fuel are:
1) Compression Ratio.
2) Forced induction (Turbo, SC, or Nitrous).
I believe the CR for a 4.0L v6 is 10.0:1, which should use 91 octane.
The v8 on...