Ihave a 2005 Subaru Outback Sedan and the owner's manual recommends 91 octane minimum. For obvious reasons I'd like to use 89 or 87 octane gas. Would there be any short term or long term damage to the engine? The specs are:

Engine Model: EZ30D, 3.0 Liter, DOHC, non-turbo.
The top of the engine is lable DOHC Boxer

Compression:10.7:1

My suggestion is try the 87 octane and see how it works. You will probably find that under the vast majority of driving conditions you will find no noticeable difference in performance. Modern engines have anti-knock sensors that will prevent any damage to your engine from using low-octane gas. However, under particularly heavy loads (accelerating up a hill) you may find that the engine seems a bit more sluggish than when using high octane fuel. It's also possible that your mileage may suffer slightly, but probably not enough to offset the difference in price per gallon.

Another option is to "blend" your own brew. Every other fill up, when the tank is half full, add the appropriate octane to achieve the desired blend.