Marion
Regardless of what finish you put on the teak it will degrade over time due to the exposure to ultra violet from the sun. That said, oiled finishes will probably look better than other paint or stain treatments and they are easier to reapply. Teak oil is a good choice but unless you're prepared to refinish your teak with it each (and every) year it's unlikely that your timber won't go grey in time. Natural (unfinished) teak/ iroko is all the rage amongst trendy architects now, you could succumb! If you decide not to oil it at the very least I would recommend that you seal any end grain with linseed oil or teak oil to prevent "wicking" of moisture into the wood.