Could be teak, it's hard to tell from the photo.

1. Avoid "exterior polyurethane" -- it's an oxymoron since poly has very little UV resistance and you'll be back to where you are in a season or less.

2. "Teak Oil" is no more an oil extracted from teak than baby oil is an oil extracted from babies. A manufacturer can make any sort of concoction they want and call it "Teak Oil" or "Teak Oil Finish." Most are linseed-oil based, some have a bit of varnish added (an "oil-varnish blend" ), and most have a lot of thinner that reduces the viscosity and simply evaporates away, leaving nothing contributing to the final finish. The linseed oil provides no moisture protection at all, especially as it's slightly hygroscopic (attracts water). These, in my opinion are interior finishes only. Too see what's really in one you are considering, search on its name and MSDS. Anything labeled hydrocarbon petroleum distillate, kerosene, naphtha, mineral spirits, and the like are thinners, often in excess of 50% of the product.

You have some choices.
0. Do nothing and let the teak (if that's what it is) naturally age to a light gray patina

1. Use an exterior finish such as One-Time that's 100% solids, UV cured. This is an in-the wood finish.

2. Use the paint base product described in the article below and get a film-forming finish.

Here is a link that might be useful: simlar thread on this topic

This post was edited by bobsmyuncle on Wed, Jan 22, 14 at 16:24