There is a product called Lube Plate that some of us use instead of greasing the hitch. Less mess and it seems to work well. I have used one for years now without problems.
Herk is correct as automatic sliding hitches do not require lubrication on the receiver plate because the kingpin box does not rotate. For a standard 5th wheel hitch you should use grease or a lube plate. Exactly what kind differs by manufacturer. Also notable is that if you use a lube plate that is too thick you may not be able to close the jaw. The king pin may need grease as well.