OK, some of what was said here runs counter to what I understood. Let's recap.
In simplified terms, octane (as I understand it) is a measure of the combustability of the fuel. The higher the number, the less combustable the fuel. In some situations, a highly combustable fuel may predetonate (aka knock). One of the ways to eliminate knocking is to switch to a higher octane.
However there is a limit to this: use too high of an octane and (as I was once told) you can get incomplete combustion -- not all the fuel consumed. This will result in a loss of power, increased pollution, etc.
So blindly saying "run a higher octane, always" may not be the best answer. Race gas in a NA 2.5 might not help, and might make performance worse, for example.
How am I doing so far?
So my concern is how high an octane is too high? How low is too low? And what is the "sweet spot"?
The book says a minimum of 87. It is possible that this recommendation is optimistic and works only because of a knock sensor pulling timing. But I don't know if that is true. I don't believe that the BD has a knock sensor!
Because I have no actual measurements or ways to obtain them, I have to go by my very unreliable and uncalibrated seat of my pants. My butt-o-meter says that 87 isn't good. On 89 it ran very well. On 93 it feels like it is stumbling occasionally.
I need to try buying from a few different vendors as well because maybe I just got bad gas (for the 93). I really did like how well it ran on 89 for one tank, and I have 4 months of experience with buying from various gas stations and running 87 to compare it to.
Does anyone think there would be any advantage to my resetting the ECU when I switch octanes?
I'm glad I don't hang in the NA or Service forums if they are as described. That's why I posted in this generally friendly forum.
Thanks, and keep those responses coming!