I am with DH where WD40 is concerned. Everytime I hear someone mention spraying WD40 on ignition systems, I cringe. It may work initially, but dried WD40 attracts moisture causing problems in the future. Then people just add more to cure the fault they initially caused with the same stuff!
Although WD40 can act as a penetrating fluid, it aint brill. Plus gas, mouse milk or other very low viscocity penetrating fluid is much better as they have better creeping qualities. So for those really rusted fasteners, WD40 may not be sufficient.