Hi, Margot. This is a very complicated question because teak is so unlike our domestic hardwoods. One thing that I suggest you do NOT do is grab the first can of "teak oil" on the store shelf. There are at least 3 variations of products sold as "teak oil", some of which will cause you more problems.
In my opinion, your best bet is to buy or borrow a copy of Bob Flexnor's book Understanding Wood Finishing. Chapter 4 has a lot of information on this subject, too much to copy here. If you can't find this book at your library, it is available at most major local and web booksellers.