This is what I posted in the Commander forums to a guy who posed the same question.
In most computer controlled engines the ECU is set to 87 octane unless otherwise stated. Some vehicles require premium fuel and will be indicated in the owners manual. If the vehicle is designed for 87 octane then paying for a higher grade is a waste of money. The higher octane is not a cleaner or better fuel, or will make your engine run better, or give you more HP. It is a simple waste of money.
Most ECU's will not adjust for engine timing based on the octane of the fuel. Example, if you put 91 octane in a engine designed to run 87, the ECU will still recognize it as 87 and won't adjust for the 91. Again, the reason why it's a waste of money. Now on the flip side, if your engine is designed to run on 91 and you put 87 in it, you can have problems like lower HP and pinging. I seriously doubt that the Commander has a ECU will adjust for higher octane rating. Most 50 thousand dollars vehicles can't do that.
The Commander manual says a minimum octane level of 87, which is the recommended octane level, and is the value that is required. Therefore anything higher, and your just giving your money to the oil company. Now when someone cracks the Commander ECU and can change the value from 87 to 91, then and only then will there be a benefit.
Please understand that higher octane does not equal more power. Octane is merely resistance to preignition caused by higher compression or excessive temperature. Reg 87 octane has just as much energy as 91+ in todays computer managed vehicles. Any modern engine gets just as much power from regular as compared to the higher octanes, unless your engine specifically requires it. Higher octane is merely a marketing ploy to play on the ignorant.
Don't get me wrong. There are different quality's of fuel. Different brands have different additives that may be better for your engine. I'm strictly talking about the octane rating. If you buy Shell gasoline, and one pump says 87 and the other says 91, there is no difference in the quality of that gasoline, just the octane value. So yes, definitely use good quality fuel, just don't get hung up on the octane rating. It's just like oil. There are many 5w-30's out there spread out over different brands. Is one better than the other, yes, but the weight will still be 5w-30. Lower octane does not mean "cheaper" gas.
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 HP 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. In modern ECU controlled vehicles the computer will set the octane value in the computer to not allow any variances because of the engines compression.
