same here (and not a stupid question because Iwata has a separate section on not using WD40 in the airbrush instructions that came with my HP-CR, obviously it's been asked a lot) My thought is that when using enamels and lacquers we use thinners that cut through petroleum based products, so it would cut through the WD40, and acrylics use alchohol in their thinners a lot which would have the same effect, and especially with acrylics I would hate to see what WD40 would do to the paint.
--edit--
Ok I just found those Iwata instructions - it says:
"DO NOT use light machine oil or WD-40 for lubrication. This will cause the needle to stick as it moves through the needle packing"
hmmm didn't think it would be as drastic as that... glad I reread it.