I think the grade of the washer has to with it's resistence to deflection. The whole purpose of the washer is to distribute the force. If it bends too easily then the force will be localized to the center of the washer, which is the same as not having it there at all. At least that's my theory until somebody corrects me. Also, I prefer a spilt lock washer, or even better a Belleville, over a nylock. I've have too many bad experiences with nylocks galling up on me.
Ah' there is a wealth of questions in asking. Cone or Belleville amongst them or was this designed for lock washer, zinc or bronze if any lock washer at all. Cone, Belleville, serrated or not, internal, external, or spring lock washer in being it's own subject beyond ferrules and or expansion and contraction or strands of wire and wire type in stranding type settling.
As said, whole wealth of concepts.
No threadlocker at least in concept if in theory conductive type.