On recipes that require melted butter, would I be able to substitute it with canola oil? I know when making pastries (pie crusts and such), cold butter is necessary for flakiness, so substitution in these situations wouldn't work.
Unless this is a very persnickety recipe, go ahead and do it. I've made the same substitution occasionally when I need to go non-dairy or when I'm just lazy. I wouldn't do it in a recipe where the flavour of butter is predominant, but otherwise go ahead.