I was going to go with the timber oil suggested earlier but the guy at my local paint shop said he had people coming back complaining of the wood turning black from mold and said it wasn't the original formulation any more. Figures. I did just end up going with a varnish called Exterior 450 and I guess I'll have to give it a new coat every year or two. I'll probably build a second similar bench in the next year and I'll try an oil on that one.