1. Mix the chipotles in adobo, oyster sauce, honey, mustard, fish sauce (if using), garlic and several generous turns from a peppermill of freshly ground pepper in a large bowl or zip-top marinating bag. Add the pork shoulder and massage the marinade into the meat. Cover (or close the bag) and refrigerate for at least 2 hours, or overnight.
2. To bake the pork shoulder, remove it from the refrigerator 1 hour before cooking.
3. Preheat the oven to 325ºF (165ºC.)
4. Place the pork shoulder in a Dutch oven or similar pot with a lid and pour any marinade over them. Add the beer, cover, and bake in the oven for about 3 hours, turning the pieces of pork over every 45 minutes.
5. When the meat is tender (depending on your pork shoulder, it may take a little more time than 3 hours), remove the pot from the oven. Increase the heat of the oven to 400ºF (200ºC.)
6. Use tongs to pull the meat off the bones, and discard the bones. Put the pot back in the oven, uncovered, and roast until the sauce is thickened reduced to your liking, turning the pork pieces a few times while roasting. When the pork pieces are gently caramelized, about 30-40 minutes, remove from the oven and serve.