hexgirl, it's an Xterra, not an X-terra
As for gas, you should get better power and mileage with good old 87 octane. It burns more readily and more completely than plus or premium with the compression ratio and timing of the engine. The engine cannot advance the timing to potentially take advantage of higher octane fuel.
If it isn't knocking, you're wasting your money putting anything more than regular in there. There's no "babying" it here.
Brent