Preheat the oven to 450°F. Place beef in roasting pan. Brush with the oil; sprinkle and rub with chili powder, oregano, cumin, salt and pepper. Roast 20 minutes, and then lower oven temperature to 325°F.
Finish cooking the beef to desired doneness, approximately 30 minutes more cooking time for rare, and 30 to 35 minutes for medium-rare. (The best way to gauge doneness is to use an instant-read meat thermometer. A rare roast will be done at 120°F; a medium-rare roast will be about 130°F.) Set cooked roast on a plate, tent with foil and rest 10 minutes. Make dipping sauce by setting the roasting pan over medium-high heat. Add the stock, barbecue sauce, beer and mustard. Simmer this mixture 5 minutes.
Thinly slice the beef, set on buns and drizzle meat with a little of the dipping sauce. Serve the rest of sauce in small bowls for dunking.