In 10-inch skillet, heat oil over high heat. Cook pork roast in oil 6 to 10 minutes or until brown on all sides.
In 3- to 4-quart slow cooker, place pork roast, 2 teaspoon salt, the pepper, onion and 1 cup of the root beer. Cover; cook on Low heat setting 7 to 8 hours.
About 30 minutes before serving, in 4-quart saucepan or Dutch oven, heat remaining 2 cups root beer, the chili sauce and 1/2 teaspoon salt to boiling over medium-high heat. Reduce heat to medium. Cook 8 to 10 minutes, stirring occasionally, until mixture is glossy and reduced to 2 1/2 to 3 cups.
Remove pork from slow cooker, discarding cooking liquid. Shred pork, using 2 forks to pull pork apart. Stir in chili sauce mixture. To serve, spoon about 1/2 cup pork mixture in each bun.