Here’s another vote for a good quality exterior primer and paint. They come with a lifetime guarantee and are definitely the longest lasting option and relatively easy to maintain.
Clear finishes on wood don’t last as long as paint, though some do last much longer than others. There’s a wide selection of finish types and a wider range of performance. One good source of information on the performance of exterior finishes is the wooden boat community. They run long term tests on all kinds of exterior finishes ranging from wipe on oil finishes (e.g., Teak Oil) to catalyzed polyurethanes. Note that some of the exterior finishes contain a lot of pigment to block the UV and are called stains.
I’m attaching a test report from Classic Boat magazine showing the results from two different test sessions. The chart box on the left lists the finishes they used for the first round of tests that started 11-1/2 years ago (from date of publishing). The chart box on the right is from the third round of testing and show the results after 5-1/2 years.
The winner from the first test is Colean polyurethane. It cracked at the sharp corners of the wood, but otherwise held up exceptionally well. Rounding the corners of the wood will help prolong the life of your exterior paint/finish. The winner from the second test is Awlgrip Alwood MA. Both products are 1K moisture cure polyurethane. Either of these finishes would be a good choice for longevity.