...on very nice bikes, such as the one you describe, it is not unusual to see more corrosion
along the seat tube/seat post interface due to the combination of regular atmospheric effects
and the galvanic difference between the (usually) aluminum alloy seat post and the (usually)
steel seat tubing of the frame. I encounter this all the time here in NorCal.

So I routinely treat the interiors of all the tubes I can get into with this stuff I get at Home Depot:



I've read the material safety data sheet for this, and also for the Amsoil product,
and they seem similar enough to me that I'm comfortable recommending and using it.

It has to dry for a couple of days after application, and you really ought to strip the
bike of all components in order to apply it easily and thoroughly.

I am also a believer in the use of anti-seize grease on dissimilar metal surface contacts,
which includes seat posts and small machine screws that go into alloy parts. Not everyone
agrees, and I have learned not to argue the point here on teh Biekfrorums.