Plot hole: If we assume that Dexter took off the oil cap while it was still in the shop for Arthur, it still doesn't explain how Dexter got the jump on him when his car (hours later) broke down. Arthur's not shown to stop, but to merely drive for hours on end until night. Arthur's on a barren strip of road with no other cars or traffic to be seen for miles. It cuts from Arthur looking in his engine to Dexter suddenly walking out from behind the Mustang. The only explanation is that Dexter waited in the car with Arthur, which doesn't make very much sense. All this takes place in the last 15 minutes of the finale.