Season the flour with salt and pepper; dredge the crabs in the flour, tapping off any excess. In a large skillet, melt 1 tablespoon of the butter in 1 tablespoon of the oil. Add 4 of the crabs to the skillet and cook over moderately high heat until golden, about 3 minutes per side. Transfer the crabs to a low oven and keep warm. Repeat with another tablespoon each of the butter and oil and the remaining crabs.