My Isel is 25 years old and re greased it recently! Avoid car grease, it will spin off the screws and just fling everywhere (car bearings are relatively loose). I was lucky and got hold of a 2 litre pot of grease from work specifically for ball screws. You need a grease that is for high load applications because the balls in the bearing are probably pre loaded and you don't want the grease to squeeze out. Mine have no nipples, I just wiped the screws and ran them round a bit. A significant reduction in noise! I just looked it up though, ISOFLEX NBU15, and it's £48 for tiny a little tube!!!!!! I suppose I'd better take the "borrowed" tube back! There must be cheaper equivalents though I'm sure.
Tom