What is the Fumoto Drain Valve? It is an alternative to the OEM oil sump plug. Why do you need an alternative? The OEM is made of cheese and there are 3 million known instances of owners stripping it and enduring untold misery. Furthermore, the Fumoto is a "quick-drain" valve, meaning you drain the sump with just a flip of a toggle, no tools required.
Here is some history. If you dont have a 24mm socket for your new plug, 15/16 fits even a little more snugly. Best to use a hex rather than an octo and grind down the bevel to have more purchase. If you have not done any of these and have managed to round your plug, here is what it should look like:
Here is what a Fumoto looks like (courtesy: DLaydApx & Pokes):
The CS uses the same OEM plug as the GS. The right Fumoto to get is model # FG-6.
Dlayd and xxaarraa figured out that unless you use 2-3 crush washers with the Fumoto, it will seep oil. Be sure to use the washer that comes with the Fumoto (fabric) in conjunction with 1-2 metal OEM washers and you are good to go. Proven no leaks in many thousands of miles.