Interesting info.. And for the record.. the LR engine is considered a "high performance engine" and operates at a higher temperature.

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.
The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

 

Terms may be misleading. The 4.0 engine, in most of the NAS market, is 9.35 compression ratio, look right above engine serial number. Back when you you could buy "real" gasoline in the 60's, some fortunate Mopar guys had those funny domed head motors with 12:1 ratio. Aftermarket might go to 14:1. A 426 HEMI was a beautiful thing, and no one worried about gas prices.

So while it is higher than some, it is hardly what old guys would consider high performance. High maintenance, yes.

Now I can show you what cheap gas also does, in these pix of a 180K+ engine, I'm sure that most of the life was on regular.

The knock sensors talk to the ECU and it adjusts timing so not so much knocking and pinging,. I think most people won't hurt the engine running regular, they may not get the best mpg. Mountains vs flat lands could be an issue.

 

Hey, if you run 87 octane you will easily loose nearly 2 MPG and and some low end torque besides the fact you can carbon up the valves and stem. Sure, as Buzz said, the knock sensors will adjust the timing to clear the knock, at the expense of MPG and power.
I dare anyone on this thread to prove they haven't lost and MPG or torque.
Run the highest octane you can and enjoy your truck.

 

Also consider that what sells as premium today is not the 100 octane of old. Seem like Sunoco had those blending pumps where you could get 102..... my oldest brother tanked up Dad's 421 dual 4BBL Bonneville with 136 octane aviation gas, not a good idea.

 

When I rebuilt my 1991 Range Rover classic, with 3.9 same engine, basically, as all of these GM derived aluminum block 215 engine, I installed lower compression ratio pistons, SO THAT I COULD USE REGULAR CHEAP GASOLINE AVAILABLE TO US ALL TODAY, WITHOUT DEALING WITH PINGING PROBLEMS. I ran it for over 100,000 miles that way, before trading it off on a 2004 Discovery. Yes, it cost me less mpg, somewhat less power; less torque, but at the gas pump, it cost me far less. I question just how much power and torque is needed by the average Rover driver, and I seriously doubt the average guy will even notice any loss of power or torque, but he will seriously notice the difference in gasoline prices over the life of vehicle. Too many guys place too much concern over power/torque loss, when they would be better served by placing more concern over getting from point a to point b for less gasoline price.