Slow Cooker Texas Pulled Pork

Ingredients

• 1 tsp vegetable oil
• 1 (4-pound) pork shoulder roast
• 1 cup barbecue sauce
• 1/2 cup apple cider vinegar
• 1/2 cup chicken broth
• 1/4 cup light brown sugar
• 1 tbsp prepared yellow mustard
• 1 tbsp Worcestershire
• 1 tbsp chili powder
• 1 extra-large onion, chopped
• 2 large cloves garlic, crushed
• 1 1/2 teaspoons dried thyme
• 8 hamburger buns, split
• 2 tbsp butter, or as needed