Grease helps parts move and protects contact surfaces. Grip-enhancing assembly paste works differently: fine particles increase friction between clamped surfaces. One application is the seatpost-to-frame interface, where the particular component manufacturers permit it.
If a post slips, first check its size and compatibility with the frame. Paste cannot make a post of the wrong diameter fit correctly. Then follow the frame and seatpost instructions for surface preparation, insertion depth and tightening the clamp.
For a steel or aluminium post in a steel or aluminium frame, grease or anti-seize can protect the joint against corrosion. When carbon is involved, the choice depends on the instructions for both parts. One recommendation cannot automatically cover every material combination.
SAC-2, for example, is designed to increase grip at seatpost, handlebar and stem clamps. Its silica particles may leave cosmetic marks. It is unsuitable for bearing lubrication or threads. Paste does not justify exceeding the specified tightening torque; a correctly assembled post that still slips needs a workshop inspection.