we use starbrite tropical teak oil.
when we bought our csy 37 B plan ten years ago the teak had been ignored for years. we gave the teak a thorough cleaning and then applied two coats of starbrite tropical teak oil. four years later we washed it and applied two more coats. four year later we washed it and applied two more coats. been two years now and it still looks pretty good.
i wouldn't use anything else. it's basically clean the wood and brush it on. no sanding or other special prep. i can't believe everyone isn't using this stuff...