My personal opinion is don't use grease inside the speedometer. I did when I rebuilt mine long ago. In a couple years the grease dried out and got hard. A speedometer may look sealed pretty well, but I have a little very fine dust make its way into mine. I think it helps harden the grease over time.
One day while driving the speedometer started making a load noise with the needle bouncing erratically. I pulled over immediatly and disconnected the cable. Only one small geared part in the speedometer was ruined, I was lucky. I had a junk speedometer laying around and replaced the part, I suppose nowadays I might have to buy the whole head to get a replacement part.
I would use a little light oil like 3in1 or maybe a dry lube spray with teflon.
