Saute the onions in the oil until soft. Add garlic, chili powder, and pepper and cook another minute or so. Add chili sauce, brown sugar, vinegar, and Worcestershire sauce. Heat till boiling stirring constantly.
Put the pork shoulder in the crockpot and pour the sauce over it. Cover and cook on low 12 hours or on high 6 hours, until pork is basically falling apart. (Longer never hurts.) Shred the meat with a pair of forks and return to the pot and keep warm in the sauce. Serve over buns.
This makes a LOT of pork. I typically freeze the leftovers and they reheat just fine.