Linseed oil by itself, or shellac by itself would be a really poor choice.
Since it is a work bench, I would use what ever I had on hand, but I have a woodshop with quite a bit of finishing materials. Usually for a work bench, I use a mixture of linseed oil, spar varnish, and mineral spirits, but spar varnish by itself would be fine. I just don't like it going on thick, so I thin it out for the first coat or two, then use just the varnish for the final coat or two.
