If it were me I would use a water based polyurethane. An oil based polyurethane will yellow as it ages and if you are not staining the wood that would be a issue in a few years. You might try some samples and see if you like it. A water based polyurethane has a really bland look to it. To show up the grain better you could coat the wood with linseed oil first but you should wait three days to a week before using a water based polyurethane over the top. If the linseed oil isn't completely dry it can cause adhesion problems with the finish. The water based poly doesn't brush very well though. It really needs to be sprayed. It can be easily be done with an airless sprayer or a conventional sprayer with a pressure pot.