C3040S is an OEM style sending unit for the Shell Valley fuel tank. It has a built-in fuel pick up and has a 0-90 ohm rating.
C3040F is a VDO adjustable depth fuel sender with a 10-180 ohm rating.
If using the Shell Valley fuel tank, you will need part #C3040S for the fuel pick up. If your fuel gauge is anything other than 0-90 ohm, you will also need to add an additional sending unit such as the VDO unit to run the gauge.