5th wheel grease is a heavy-duty graphite EP2 product formulated for the coupling plate between a truck tractor and its trailer.

The fifth wheel is an unusual lubrication problem. The trailer skid plate slides across the coupling under enormous vertical load every time the rig turns, with no rolling element to help, so the grease has to carry very high contact pressure while resisting being wiped and squeezed out. Graphite is included as a solid lubricant that stays in place and keeps working even where the oil film is pushed out of the contact.

Apply it in a proper pattern across the plate rather than in a single blob, keep the release mechanism clean and separately lubricated, and check the plate at every service. A dry fifth wheel wears both plate and skid quickly and makes the rig heavy to steer.