Carbon rubbing against copper (or silver or whatever is used) makes a pretty good low resistance electrical contact, I dont think I would like WD 40 to come between or interfere with or add resistance, or even conductivity where it may NOT be wanted.
HOWEVER (1) There is something to be said to reducing the coppers (or silver or whatever material is used) reaction with oxygen (oxidation) which some sort of temporary surface coating could reduce and THEN REMOVE IT PRIOR TO USE but
HOWEVER (2) As soon as the genny spins, the carbon brushes should quickly polish off any oxidation that may have occurred during periods of setting !!!
SO I DONT THINK I WOULD USE WD 40, maybe some sort of electrical contact cleaner if and when necessary
NO WARRANTY chemistry and material science arent my cup o tea. Hey WD 40 can provide a coating which reduces oxidation and it has cleaning properties, but I just dont like the sound of what that old timer said fer some reason ..............
Ol John T and all