Slow Cooker Texas Pulled Pork

INGREDIENTS

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