Exterior finish Help!!!!

Hi, I am new to woodwork and am starting my first project.:no: I am building a letterbox from a piece of ironbark from an old timber bridge. I was wondering which is the best exterior finish to use for this. The finish isn't going to be smooth due to the age of the timber, but I want it to last and retain the lovely red colour of the timber. I am not keep on having to recoat it every 12 monthsI am thinking along the line of an oil but would value anyones input.
Cheers:doh:

Decking oil would do. A good quality one (ie sikkens) would only need recoating every three years or so I guess.

Cheers.:U

i'll let you know how i go. will post pics upon completion.