I used Teak in my truck. Used 10 coats of a UV Spar Vanish from West Marine (boat store). It looks great and I just wet sanded the finish and buffed it out after 1-1/2 years, look perfect.
With Teak, you can leave it natural (of which it will turn a silver color in the sun), you can oil it, or you can varnish it. Just don't use a plastic varnish like varathane, it is not flex with the wood....
Before the cutting...
During the varnishing stage....
Almost done...
And in the truck...