standard household 3-in-1 oil works great, as does good ol engine oil.
in a production environment, i'd change my tune because you are counting on tap life. for hobby purposes, you'll never notice the decrease. In our case, it's there to keep the aluminum and mag from sticking to the tap, locking it in the hole and making for a very long day. Don't sweat it, even some veggie oil or Crisco would do the trick in a pinch.
Speaking of Crisco (the white goop, not the liquid), it makes an excellent lapping compound when you're trying to get that last fraction of a micron out of something. we use it all the time at work on machine components at work that are just a touch oversize for our assembly preferences. Arm and Hammer toothpaste is also great if you need to be more aggressive with it. Machine shop hacks 101 hahahaha