Just rub a soft pencil over the nut slots. Graphite powder will fall into the slots and help lube the nut. I think proprietary nut lubes are quite cheap these days but you can make your own by sanding pencil lead and mixing it with some petroleum jelly (Vaseline in the UK). Make a paste and apply to the nut slots with a toothpick or similar. I've never used graphite on a bridge, not sure how effective it would be.