Do I need to put an emulsion or other on the rubber piece between the toilet tank and the toilet?

I removed the tank of my Toto Drake toilet so I could paint behind it. The black rubbery part that goes between the tank and toilet was sort of wet and slick. My question is was this normal or do I need to put some emulsion like glycerin or what on it? Does it need something like that or can I just put it back dry?