Grease the bowl of a 6-quart slow cooker with vegetable oil.
Season all sides of pork roast with seasoned salt and place into the greased slow cooker bowl.
Place chopped onions over seasoned meat.
In a medium bowl, whisk together root beer, liquid smoke, barbecue sauce, brown sugar, mustard, Worcestershire sauce, chili powder and garlic. Pour mixture over pork roast and onions.
Cover and cook on high for 4 hours or on low for 6 to 8 hours until fork-tender.
Carefully remove pork from slow cooker and shred with two forks. Return shredded meat to slow cooker and stir all of the juices and meat together.
Spread softened butter on hamburger buns and toast butter-side up under oven broiler until golden brown.
Spoon a generous amount of pork onto toasted bun bottoms and top with the other bun half. You may also top meat with coleslaw or additional onions and pickles.