Convert variable input formats to black and white MP4
Yes, you do need a ffmpeg build with gray pixel format enabled. You should get your binary from the Windows link at ffmpeg. Download the latest 32-bit static build.

You can check if your build has gray enabled by running 

ffmpeg -pix_fmts | findstr /r gray


It should list an entry for gray. 

And use format=gray in your vf chain and -pix_fmt yuv420p together for broad compatibility.


  The command below is for linux. grep will work as findstr!

ffmpeg -pix_fmts | grep -i gray