Can not see how adding grease will hellp with what is a friction issue.

Evillv stopped this problem on his merc by drying the tube , I stopped mine on my merc by cleaning with alchol and roughing area, and added the brommie self locking clip.
On my preivous bromie I replaced the plastic trim and roughen the seat post. (Only in the top area where it was clamped).

I had problems recently getting a dahon andros stem to hold bullbars on my brommie. Sloved this by switching from chrome to anodised metal.

I have noticed how prone to rust my new brommie seatposts is also.