I have a ton of 15w40 oil left from my previous car. I am assuming it will be ok and I will cause no harm to an STI using 15w40 oil. Am I correct in my assumption?
Ryan

While I'm a big advocate of using the "correct" grade of oil, I'd say you will be fine if you are doing so just to get rid of oil you have left around. 15W40 is a little thinner than the recommended stuff, but since you live in AZ, I don't seen any harm in using what you have to get rid of it.

No, not correct. There is a viscosity chart in your owners manual with recommended viscosities for expected ambient temperatures, that is what you should follow. 10W40 is the maximum viscosity Subaru recommends.

edit- uh-oh, mod fight.
It's actually thicker not thinner, and while it may not actually do any harm it is best to stick with the recommended oils.

How about if I mix 3 quarts of 15w40 and 1.5-2 quarts of 10w30, will that be better than running full 15w40?
Ryan

Rather than pouring a concoction in there you'd be best off just using the right oil. That mix will result in an unknown viscosity that will still be higher than it should be, plus you'd be mixing different additives and other stuff.
I'd think that the main issue with the 15W would be too much pressure on cold startup, maybe less of an issue there in AZ but still a consideration. Heavier oil will also add drag on the engine and cost you some HP.
I realize that you may have a lot of this oil on hand but IMO after spending $30K on the STi it's worth a few more $ for the correct oil.

I guess I will trade my dad some of my 15w40 for some of his synthetic 10w30. I might mix 1 quart of 15w40 w/ my oil changes for a while. 1 quart shouldn't hurt anything.
Ryan