I have a dry rub I use that's pretty simple and goes good on pretty much any bbq.
1/2 cup brown sugar
1/4 cup paprika
1 tablespoon pepper
1 tablespoon salt
1 tablespoon chili powder
1 tablespoon garlic powder
1 tablespoon onion powder
1 teaspoon cayenne
Rub the loin down, wrap it up, and put it in the fridge over night. I usually smoke a 5-7 lb loin and TRY to keep it between 225 and 250 for 4-5 hours. You'll obviously need more rub and possibly more time but you're on your own there. Math makes me sad.