Soak bamboo skewers in water for 30 minutes to prevent burning during grilling. While waiting, make your marinade by mixing 7-up, soy sauce, vinegar, 2 cups oyster sauce, brown sugar, minced garlic, black pepper, and minced Thai chilies in a large bowl. Stir until sugar completely dissolves.
Slice your pork butt into uniform pieces, about 1-inch wide and ¼-inch thick (pang-BBQ na hiwa). You can ask your butcher to do this for you. Rinse the meat and pat dry with paper towels.
Put the meat in the marinade, making sure all pieces are well-coated. Cover and place in the refrigerator for 4-8 hours or overnight.
When ready to cook, start your charcoal grill. Wait until the charcoal is covered with gray ash and glowing red underneath (about 350-400°F/175-200°C).
While the grill heats, make your basting sauce. Mix banana ketchup, sesame oil, and 1 cup oyster sauce in a bowl. Set aside.
Take the meat from the marinade and thread 2-3 slices onto each soaked skewer, leaving small gaps between pieces. Throw away the used marinade.
Place skewers on the hot grill. Cook for 2-3 minutes on each side before applying any basting sauce. Once the meat starts changing color, brush on your basting sauce.
Keep grilling and basting, turning the skewers every few minutes until cooked through, about 15-20 minutes total. If flames flare up, move the skewers to a cooler spot on the grill.
For the dipping sauce, mix vinegar, minced garlic, chopped onions, chopped Thai chilies, pepper, and salt in a small bowl.
Let the barbecue rest for 2-3 minutes after grilling. Serve hot with the spiced vinegar dip, steamed rice, and pickled papaya (atchara).
Enjoy your homemade Filipino Pork Barbecue while it's hot and fresh off the grill!
Note: For food safety, the pork should reach an internal temperature of 145°F (63°C).