Photoshop - saving as a grayscale PNG

[Psyk]

I'm trying to make a heightmap for a game and I need to save an image as a grayscale PNG (preferably 16 bit). I know this format exists, but whatever I do I can't get photoshop to actually save in this exact format. I've tried going to image->mode->grayscale but it still doesn't seem to be saving in grayscale format. When I try to import it into the game it complains saying it's not grayscale. How can I make sure it saves in this format?

 

[Psyk]

Ok now same question but for GIMP as well. Same problem really. Even though it's working in grayscale, it's not saving as grayscale.

 

[whitecrook]

use imagemagick to convert it. Photoshop is known to suck eggs at PNG handling.