I agree, PTFE or even silicon spray (Halfords, quite reasonable) works very well on the luff slot. I'd avoid WD40 as it will get sticky as it dries off and dirt will stick to it, making things worse. Whereas silicon dries cleanly leaving a very slippery surface.
My mast is generally left up so what I do it is to get a rag or heavy duty kitchen roll stuff, soak it liberally with the silicon (or PTFE) spray, then wedge it into the slot (not too tight) between the top two slugs of your main, when you haul the sail up and down it will then lubricate the whole length of the luff slot. Or, of course if your mast is lowered you just squirt it in!
Hope this helps.
Andy
Baycruiser23 'Equinox'