Bought the wife a used Jamis at the local. She loves it, and we are riding frequently. Each time we ride she is having issue with the seatpost sliding down into the tube. It is the original seatpost to the bike, as well as clamp. It's one of the "lever" clamps, and I have it tightened to a point I can barely open or close it by hand, and it feels as if the lever will break if any tighter.

I took it back to the shop we bought it from a few weeks ago. Shame on me, it was Friday afternoon and moments before close. I asked the fellow behind the counter if he would apply some of the "paste" to the post so it could get grip. I could see him eyeing the bike with a mind towards thinking we didn't buy it there, it's not one of their brands. Without really having time to go into it, I just wanted him to put some paste on the post and tighten it down...where he goes on to argue with me that it is designed for carbon, and won't work...that they apply white lithium grease (which explains the sliding) to keep them from seizing. Long story short we are pretty much pushed out the door without a resolution and continued to stop every few miles to raise the post again.

I KNOW there is some manner of paste we used at the shop I worked to combat this. IDK if it were specific to carbon, metals, w/e, hell it may have even been tooth paste.
Can someone please tell me if they are aware of what CAN be used on a metal to metal seat post to stop it sliding without having to buy a new part of any sort? Or just what it's called?