This is the finish I prefer....
http://www.brownells.com/.aspx/pid=5...UNSTOCK-FINISH
but most any exterior varnish / like an exterior grade Spar Varnish by Verathane...that is common in a lot of hardware stores will work just fine as well.
Varnish is way harder than any oil finish.../ but a lot of products that are called Tung Oil are really varnishes....like Watco's Danish Oil finish ( they're really a varnish as well ).