Sorry to resurrect an old thread, but I've just removed my driver's door to free off the hinges before they seize up completely, and decided to fit grease nipples to follow Hans' example.
I drilled and tapped a hole for th M6 nipples I have, but the threaded part of the nipple is about 5 or 6mm long, whereas the thickness of the cylinder part of the hinge is 3mm.
I ground down one nipple, only to find that if you do that, the spring inside is no longer held in place.
From what I can find out on Google, M6 grease nipples with a threded length shorter than 5mm do not exist.
Does any one know differently? Or will I have to space out the nipple with a short collar?