>>1751054it's universally agreed upon standard practice
grease is thicker and will last better
I don't think you'll find anyone recommending oiling a seatpost
Also it's not rust, it's aluminium oxide produced by galvanic corrosion which basically welds an alluminium post to a steel frame.
I think grease between clamped metal parts actually also facilitates tightening. You're supposed to grease bar clamps also