Here is the basis for the recipe I used for the marinade. I added ancho chiles and a dash of cumin as well.. I also forgot to mention its important to cut slits into the butt so you can get the marinade down deep into the meat..
7 lbs. pork shoulder - picnic cut (with fat)
9 garlic cloves (You may use powder garlic or ground garlic sold in glass jars - 1 tsp. equals one clove.)
1 teaspoon black pepper
1 Handful fresh oregano
1½ tablespoons olive oil
1½ teaspoon salt
Ancho chlie powder
Dash of cumin
1 1/2 tablespoon white vinegar