Our interior finish was the original oiled finish and after 20 years, she was rather dirty and dark looking. So, one winter when there wasn't anything better to do, my wife and I removed every possible piece of teak (80 some odd pieces) took them home to strip and finish those. Meanwhile down at the boatyard, we used random orbital sanders as well as hand sanding to get the teak down to bare. Then we carefully vacuumed it all up, wipe everything down with mineral spirits then wiped again with a tack cloth. Now we decided that a marine/exterior polyurethane (Minwax Helmsman) would work nicely for the interior, and it has for the last ten years. After each coat of finish, we lightly sanded with 220 grit to keep things smooth. Don't forget to wipe it down again after each sanding. We have three coats total.
Make no mistake about it, this is a big job to do it right, but I can't imagine living with that dark oiled finish. The gloss really brightens things up.
Good luck and remember that she's a classic and deserves the best