Barbecue sauce: add the barbecue sauce, Worcestershire sauce, mustard powder, garlic powder, onion powder and smoked paprika to a medium bowl and whisk to combine then set aside.
Pork shoulder: place the pork shoulder on a clean, dry cutting board and remove any packaging. Then use a folded piece of paper towel to pat it dry. Next flip it meat side up and fat side down. Unfold it and then season the meat with salt and black pepper. Next use a basting brush to coat this side of the meat with the seasoned barbecue sauce. Then roll the pork back up and flip it over so that the fat side is on top.
Slow cooker: grease the base and sides of the slow cooker insert with cooking oil. Then place the pork shoulder into the base. Open the 1st can of coke and pour this into the base of the slow cooker around the pork. Stop if the pork looks like it will be fully submerged in coke. We want the fat layer sticking out the top. Now take the basting brush and liberally brush the sauce over the top of the pork. Place the lid on the slow cooker and set the timer and temperature to 4 hours on high or 7-8 hours on low.
WITH ABOUT 15 MINUTES COOK TIME TO GO: Preheat oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5.
Baste and cook: transfer the cooked pork to a large baking dish. Open the 2nd can of coke and pour ½ of this across the surface of the pork. Transfer the pork into the preheated oven on the middle shelf. Let it cook for 10 minutes. Once this cook time is up use oven mitts to remove the baking dish and set it on a heatproof surface.
Cola glaze: pour the remaining coca cola into the leftover barbecue sauce and gently stir as it fizzles and gets absorbed into the sauce.
Fat layer: use a sharp knife and tongs to remove the fat layer from the pork. Then baste the surface where you cut the fat off and pour in the rest of the sauce. Return to the oven for 10 more minutes.
Pull and baste: after the 2nd oven bake, use the oven mitts to again remove the baking tray from the oven and use 2 forks to shred the pork shoulder.
To serve: use tongs to transfer the pulled pork to toasted burger buns with coleslaw. Serve warm.