Pistachio oil is recommended here for the obvious reason; it best complements the nut flavor. However, it is hard to come by and rather expensive so I’ve offered grapeseed as an alternative because it is neutral-tasting. If you really didn’t want to bother with buying a special oil, use canola. Also you could use olive oil - bearing in mind it will add its own flavor to the butter (although this may be a good thing, depending on your taste!)