There are many threads discussing this and just as many opinions. Soap and water are not a good thing for any wood finish. Do a search on this site. Mahoney’s walnut oil is a good choice but you’ll need to reoil periodically no matter what you use.
"Soap and water are not a good thing for any wood *finish*." I'm not prepared to defend or refute this claim, but I will say that mild soap and warm water (i.e. hand washing) are, for all intents and purposes, harmless to the *wood* itself. The finish, though, well...
Wood is an amazing substance. I heard it once said by wood scientists that they do not know how long wood can last, or rather if we can keep it dry, and not have it in an environment conducive to decomposition, wood products will last indefinitely. Wood structures have been standing for a thousand years and more all over the globe. We can wet it as often as we want, as long as it is wet only as long as necessary, then allowed to fully dry. Prolonged dampness is what sets up rot, not necessarily constant submersion. (There is an entire industry that dredges sunken logs from waterways where they've been for hundreds of years or more, and the wood is in near perfect condition. And archiologists unearth wood products all the time that have been buried in low oxygen environments and the items are in nearly the same condition as when they were in use.)
There was a local bar/restaurant near me with a south Pacific theme. One of their gimmicks was wood plates. The problem was the wood plates had an industrial plastic finish on them, a finish that was unable to withstand a constant environment of exposure to sharp knives and forks and spoons, and to add insult to injury, they were cleaned and sanitized in commercial dishwashing machines with scalding hot water and harsh chemical cleaners, including chlorine bleach-based sanitizer. After my second trip there, and second meal off a plate whose plastic-coated surface was flaking and the exposed wood becoming fuzzy, and the plates reeking of bleach, we stopped eating there.
Mahoney's walnut oil would be a fine choice for a display piece, but it serves no purpose, other than temporary cosmetics, for a piece that is subject to daily use and daily cleaning, only to require periodic re-application. Not to speak for Mike Mahoney, but he builds bowls made for daily use, not for decoration. If he applies walnut oil to a new bowl, it's to make it look pretty on a sales shelf, the oil finish serves no purpose to the bowl in the kitchen.
Let's be honest, non-woodworker recipients of the functional wood bowl, plate, or utinsel, 99% of them, are never going to re-apply oil, or any finish, as suggested by the maker of the piece. Not more than once, anyway. We will make them think, scare them to think, the piece is so precious, so fragile, that they may not use it much at all, or won't want the bother of it.
Build the bowls and plates so they have a bit of extra mass (vs. an art piece) to withstand the knocks and dings of daily life, and give them a shape that makes it easy and confident to handle, full of food, with one hand. And clean them with mild soap and warm water immediately after use, with clean towel and thorough air drying before storage. No sitting damp and food covered in the sink overnight. And no microwave cooking use. And skip the finish application. Do these things and the bowls and plates will last a lifetime. Or two.
