Heat oil in a large pan over medium-high heat. Season pork with salt and pepper on all sides. Place in pan and cook to brown all sides.
Mix together dark brown sugar and cayenne pepper. Place sliced onion in bottom of a crock pot.
Place pork on top of onions and coat with brown sugar mixture. Pour can of Dr. Pepper around the sides of 6-quart crock pot. Place lid on crock pot and cook on LOW 7 to 8 hours or on high 5 to 6 hours.
To make sauce, melt butter in a medium saucepan . Add garlic and onion and cook until soft, about 4 minutes.
Add remaining ingredients and bring to a simmer. Cook until thickened, about 30 minutes.
When pork is done, remove it from crock pot and shred with 2 forks. Discard liquid in crock pot. Return pork to crock pot and add 1/2 to 1 cup of sauce. Mix in with pork. Cover crock pot and cook on LOW for 1 hour.