Split washers (lock washer) and flat washers do nothing to seal. Split washer is used to help keep nut tight by adding a force to increase the friction of nut threads to bolt threads. Flat washer is used to keep from galling the surface of the part when tightening and to be a spacer to ensure the nut tightens on the part and not bottoms out on the bolt threads before getting to proper torque. Proper grip of bolt has no threads in sheer inside hole of parts.
A true crush washer is like used on the oil pan plug for the screen. It is used to seal. It is made out of copper and meant to crush under the head of a bolt, typically used when internally threaded and therefore no nut on something with fluid on the inside. It does not provide the locking feature of a split washer as it crushes and provides no axial force on the bolt to help increase thread friction