A teak exterior door is easy to maintain. It is only oiled after a thorough cleaning. On the other hand, it should be done every year.
If the door is lightly soiled, washing the dirt off is enough, but if the dirt is a little harder, you may have to sand it off. If the door is very dirty, it may be a good idea to “scrape” off old oil, dirt and weathering, then lightly sand it. Use a recommended teak oil and follow the instructions on the can. Be aware that cotton wool pads used to wipe off excess oil can very easily self-ignite and should therefore be soaked in a jar of water.