A tube of lithium grease from your local factors will do. They'll know what you're after. Do it a couple of times a year, more if you're off-road a lot. Not sure how many nipples there are per prop on the classic but check each joint and the middle. You'll probably find a little ball in the centre of the nipple. Sometimes it helps to push that in with a nail or something to just check it is free first. Then pump it in until all the old black grease has been pushed out and new grease is leaking out everywhere. You may have to turn the prop a bit to get an angle where you can get the grease gun in there. As ever, make sure the car is well supported and back up supported when you're under it.