Two things that will eat glow plugs quickly is running too lean (which you say you aren't doing) and bearings that are going bad. Neither of these may be your problem, but it's something to think about. WD-40's active ingredient is fish oil, and I doubt it does any good to protect your engine. You mentioned that you haven't seen much needle valve response; that sounds like air leaks. If you are leaking air, you could be going lean in flight which would eat glow plugs.