Which lube to use ? Try Vaseline, and KY, then decide. They are both entirely different, but achieve the same result. As an adult, unless you have a tiny bottom, or really tight anus, (both of which probably go together),you shouldn't need any lube. It's pretty hard to feel the thermometer in your butt, and lube seems to decrease the sensation even more. The neat thing about Vaseline, is that it's the classic lubricant, that those of us over 50 would remember. KY is kind of the clinical and modern replacement. It's almost like hair gel. Nothing sexy about it. Vaseline melts from body heat, and drips off of the thermometer....that's what makes it special !