In a previous question I asked how to remove rust from my seat tube/post. In the comments of one answer somebody said I should be oiling my seat post to prevent it from rusting into my seat tube, but I'm concerned this will cause the tube to slip when jarred.
Should I be applying a thin coat of oil to my seat post to prevent rust from forming between it an the seat tube? What sort of oil, if any, should I be using?
Greasing your seatpost will prevent problems like this: "Removing Seat Post rusted into downtube".
Don't use chain oil, grease is what you want. As whatsisname pointed out, all fasteners on your bike should be lightly greased. This is what keeps them from corroding and seizing over time, becoming a real pain to remove.
If you have trouble with a slipping seatpost (like me), carbon grease is what you need. Contains enough grit to keep from sliding, while still offering protection.
You should absolutely grease your seatpost (unless it is carbon fiber). It won't slip around if your seatpost clamp is properly tightened.
Get a thing of grease from the bike shop, like the park tool PPL-1 grease. Better yet, get the big tub.
Then, generously apply it all over inside your seattube. When you think you have enough, add some more. Then put the seatpost in and twist it around a lot and move it up and down to ensure the grease covers all of the seatposts surface.
Screw threads are often greased when they are installed, but that doesn't cause them to get loose any easier. Same thing with the seatpost.
Heavy, water-proof grease is a good idea.
Slipperiness isn't the reason for the grease. The goal here is to keep out moisture and oxygen, thereby discouraging oxidation. Aluminum oxide is especially problematic, as its molecules are substantially larger than the aluminum molecules in your seat post. Oxidation will cause it to jam very tightly.
In general, you don't have to worry your seat post slipping. The surface of the seat post and seat tube are not perfectly smooth. The "high spots" will push the grease out of the way, in to the "low spots" where you might otherwise see moisture and oxygen.
It doesn't take much grease to do the trick, since the seat post is already quite snug. Just make sure it's all covered.
In some cases it's possible for grease to get trapped and not be able to squeeze out, giving a result similar to hydroplaning. You tighten & tighten and things are still slippery. It's not likely in this case, because the seat post clamp is narrow.
For overkill, you can even use anti-seize:
