I've used all my soaps and creams in my scuttles several times and have yet to find one that didn't perform. I think the key is not to use boiling water. Hot tap water is good enough, though I realize that not all home water heaters are created equal.

Within the last few months I read somewhere that MWF likes lower temperatures so I tried it with warm (as opposed to hot) water in my G20 and had excellent results. I was had successful results using hot water too but IMO lowering the temp made a noticeable difference. Just something to keep in mind.