I've done 12 hours...not sure if 20 would hurt... or even make it better. Have had great luck with,
1 cup chicken broth
1 cup pineapple juice
1 cup apple juice
1 cup beef broth
1 tablespoon olive oil
1/8 cup garlic juice (from the jars of chopped garlic)
1/2 cup brown sugar
1/8 cup granulated white sugar
2 tablespoons Worcestershire sauce
2 tablespoons hot sauce of choice
1 tablespoon kosher salt
1 teaspon garlic powder
1 teaspoon celerey salt
pinch of chipotle pepper
Combine everything in sauce pan, heat, stir until blended. Add whatever to your own taste. Refrigerate. Then inject, being sure to move the needle all throughout the meat so there's no puddling. (Puddling. That's kind of a fun word, isn't it?)
Then, take your rub of choice, put in a small bowl, and add just enough of this nectar to make a slimy slather. Rub the butt with it. Be sure to get the stuff in every nook, crannie and crevice of the meat. (Don't get carried away though--it's just a pork butt.)
Stick that baby on the smoker with your wood of choice.
Take the remainder of the magic juice, add a stick of butter, put in metal bowl on the smoker, and baste the big ole butt every couple of hours.
Pull the butt at 197 degrees, let set for 20 minutes, pull the bone out, start pulling and serving, and watch your guests' eyes roll back in their head as they say "Omygod, that's the best pork I've ever had."