I watched a video of a dude replacing the CVT fluid on his 2015 fit today, from the K.I.S.S. guide. I'd imagine the process is nearly identical for a manual transmission. He said to buy a replacement "gasket" for the fill plug. Does this mean the crush washer, and if so is it exactly the same one used for the oil drain plug, or is it different for the MTF fluid drain plug?

 

Yes, crush washer and yes, same as the one used for the engine oil drain plug. Note that the crush washer for the fill plug is larger in diameter, so in the end you need two crush washers in total, one smaller [same as engine oil drain] and the other one larger in diameter.