Yet it seems to have stirred your BC's at least a bit...
Yes you need to run what your motor needs, is designed for, no argument. Just highlighting the maybe not so obvious reverse ratio of energy vs octane.
To recap; in thinner air your peak cylinder pressure will be lower, effectively the same as lower compression. So lower octane should be fine, as a benefit you will likely see higher mileage. That may also be due to reduced RH twisting; impressive scenery, rocks in the road, fewer passing opportunities, and the serious consequences of a missed apex in the mountains!
While looking for confirmation (hopefully
) of my thoughts on this I ran into a guy that did testing with an older civic and he was able to document to his satisfaction better fuel mileage with low vs high octane gas. The Prius tech forum also found that correlation and they are crazy about MPG. For whatever reason Prius tend to run poorly on high octane fuel.
Octane vs energy is very complex and fuel formulation is all over the board these days. I have a feeling that petro engineers are very good at meeting fuel specs while using what ever part of the cracking process is plentiful.
All true, but not very helpful. You can put plutonium in the engine, if it can't get the most off it, it doesn't matter. The lower octane fuel does have more energy content, but this engine is designed with more compression ratio. Conversely, an engine designed with low compression ratio will not get extra power by using higher octane fuel.
But regarding mileage, I never thought about how it would be affected by the octane. Mmm... food for thought. That hurts.