Any good lithium based wheel bearing grease will do. I happen to have Valvoline in the shop right now but have used everything from Lubriplate to Mobil 1.
Even if you use a plastic lube plate, the pin and jaws still need to be lubricated as do the pivot points on the firth wheel. If you have a trail air pinbox, don't forget the zerk fittings INSIDE the unit. Most people just lube the zerks they see on the outside.
That is the reason I don't use a lube plate, I'm already there greasing everything else, might as well use grease on the plate as well.
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