I use rapeseed or sunflower oil for things like salad bowls, mortar and pestles etc. One they are safe, two they are easily refurbished by the eventual owner and three they are cheap. Olive oil goes rancid in time so avod that. Walnut aoil, as George points out can be a problem for anyone with a nut allergy. As far as looks are concerned if it is a utilitarian piece then getting a lovely gloss or shine etc is not really a consideration as it will quickly lose it when used.
pete