the anti corrosion benefits of a thin [but frequently renewed] layer of grease helps keep the aluminum post from
seizing over time in a steel seat tube..
put the original bolts back in , then .
the force of which you speak suggests the fit of post in frame could be mismatched,
reamed out a bit large, and so the post is Not quite right.

Carefully hand reamed seat tube takes time, mass produced bikes dont get that, [time=$]
but a precise fit wont need much force to hold the adjustment.

maybe a Ti post wont need grease..

or go back to the old technique of taking the saddle off. and taking it in with you..