The best oil to protect exterior wood is Teak Oil.
Teak oil provides excellent weather and water resistance and also helps teak wood (most outdoor furniture is teak) keep its lovely honey patina.
The primary use for Teak oil has traditionally been for exterior wood such as outdoor wooden furniture and for wood on boats.
Teak oil is designed to slow down the natural greying process of Teak whilst protecting it from weathering.
Teak oil has a shelf life of five years or more and should keep in a tightly closed full or nearly full container in a cool dry place.
If Teak oil is not stored well inside and contaminated by water, or any other chemical substances, it'll probably go bad easily.
Linseed Oil is also a good oil to protect exterior wood.
Linseed oil, also known as flaxseed oil, is one of the most popular wood finishes in the world.
Like other hand-rubbed oil finishes, linseed oil saturates deep into the wood grain to protect against scratches and changes in humidity.
Linseed Oil is a great protectant for wood both indoors and outdoors.
It beautifies any wood and, once cured, protects the wood from sun and water damage.
It can really reinvigorate old dried out wood and bring it back to a healthy status.
Tung Oil is also another good oil to protect exterior wood.
Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree.
The oil and its use are believed to have originated in ancient China and appear in the writings of Confucius from about 400 BC.