If the filename is lowercase, i.e. sdl.pas, then it makes no difference whether I use SDL or sdl in my uses clause. If the filename is uppercase, i.e. SDL.pas, then I need to use SDL in my uses clause (using sdl will return an error). So I'm glad you switched to lowercase for the filenames, so that silly duffers like me can not worry about these things.

I might look through the FPC manuals to see if they mention this.