Pixelmator : how to convert to grays only?

in Mac Software edited January 2014
How do you convert an RGB (colors) pictures to grays only, in Pixelmator*? (or with preview?)

I need the document to be 8-bits gray levels only, in PNG format, not just grays in an RGB file. This is basic in Photoshop (I don't have it on my computer).

I'm suspecting there isn't an option to do that with Pixelmator and Preview (wtf !?)
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