I have used my truck to tow around 5000K miles in the past 6 mo's. I have used 87, 91 and 93 octane fuel. The manual for my 08 AC 4.0 says to run it with 91 or higher but it is okay to run other octanes. Here are my observation.
87 oct. pulls good but it must be in 4th gear all teh time and needs to downshift to 3rd on inclines. average mpg is 10.4
91 or 93 oct. pulls noticeably stronger and does not downshift into 3rd on inclines unless it is reaching 5-8% incline at highway speeds. Furthermore it can run in 5th gear on relatively flat runs locked in at 2000-2300 rpms @ 55-65mph with little or no hunting. the higher oct also gives it 11.7mpg.
So, in My experiences, Higher oct has more power, lower running rpm, and better mpg. Seems like a no brianer to me to run 91 or higher when towing or heavily loaded.
Here is a pick of my loaded rig. TT- 4000lbs, Truck cargo with passengers/dog around 1000lbs