Sorry, we're not going to add this one to the site. It's just one of those things that comes up very rarely, so we don't want people to have to make a decision about it every time they add a fuel up.
Some of the questions we ask ourselves when we consider new features are listed
here in the FAQ, and I don't think this one passes the first two: not enough people will use it, and it will complicate how people use the site now.