On page 13-13 Revised 2016 RGJG, in the italicised comments at the very end of the running board section, it mentions an anti- squeak webbing used between the running board and bracket, and I quote, "on some vehicles".
Not terribly helpful. Optional?
Lots of us have found anti squeek in the form of friction tape on top of the brackets. Get your old hockey sticks and you may find a roll. Also in the groove thr RB fits in. I use two pieces of the tape overlapping on top of the brackets. Used on my 2 FP vehicles and will again on my 3rd.
Great point, all of the anti-squeak that I have encountered (frequently between metal and wood) was friction-tape consistency BUT was saturated with some tar-like substance. I may experiment with butyl caulk in some applications I need to address.
One of the more interesting places I found this anti-squeak was as a bridge, placed against the glass rear window of sedans, across the gap in the rubber gasket that surrounds the rear glass. I doubt a better seal could be devised today.