I understand that the higher the octane rating, the slower the fuel burns. The lower the rating, the faster the fuel burns. Performance cars with higher compression engines require high octane to burn slower to prevent pre-detination we all know as the bottle rattling noise or pinging.
When driving in high altitude 5000 feet and higher, the oxygen in the air is thinner. The fuel is formulated accordingly with less octane for the fuel to burn better with less oxygen. So there is a logic to the lower octane at high altitude. It is not a gimmick. It has been that way for as long as I can remember.
BUT.......
In the case with our Ford V10 gasoline engines, the owners manual says to maintain 87 octane even in high altitude conditions. In my 2007 owners manual it is stated so on page 228 under "Octane Recommendations". This coincides with your experience. It also says that your engine sometimes will ping lightly which is considered normal. Excessive pinging is cause to see a mechanic.
So your advise is "Right On".
Pay the money and always use 87 octane fuel.
On a related note, never use a higher rated octane than your engine requires. This because over time, carbon (black soot) will build up on the valve seats which in turn causes pinging. It's sort-of-like getting your engine addicted/dependant on high octane fuel. So if you have consistently run premium fuel in a regular octane engine and suddenly switch to regular fuel as stated in the owners manual, the engine could ping a lot, forcing you to return to premium fuel.
There are decarbonizing products on the market which can sometimes be effective in fixing the addiction, pending the severity of the carbon build-up. Generally speaking fuel additives are less effective than the type that gets sucked in through a vacuum line off the intake manifold. I had very good luck with OTC D-Karbonizer on a Pontiac Fiero-GT I once treasured. I bought the car used and it pinged unless I used premium fuel. After the treatment, I was able to use regular gas as the owners manual says to. The engine seemed to have a bit more power, and of coarse my fuel costs were lower.
The OTC D-Karbonizer is a pint sized bottle of special liquid that you allow the intake vacuum draw in the liquid over the coarse of a minute or so, per instructions. When the fluid is all drawn in, you quickly turn off the engine and allow the liquid to soak into the carbon soot on the valves for a certain amount of time. Then drive the vehicle for so many miles. The D-Karbonizer liquid loosens up the soot and driving the vehicle cleans it off quickly. It's sort of a crazy process because when the fluid is drawn in, the engine wants to die so you need someone in the driver seat with the gas pedal to maintain a certian engine rpm. During this time, the exhaust billows out white smoke. It's scary but it works. Safe for catalytic converters, sensors and all.
