From the stuff I've read, they said to use a BBT thermometer. I've seen that there are actual digital and old fashioned ones, but they say "BASAL" on them. From what I've just read though, it says the main difference is that they're more accurate than just a regular thermometer (digital or mercury).

Look in the packaging for your digital thermometer and see if it too states that mouth/armpit temps vary by 1.0 degree.