Oil, varnish, or polyurethane finish on gunwales is more than cosmetic. Ash has a pretty open grain and bare ash will over time soak up moisture, split, turn gray, and eventually rot without it. But if you store your boat in a protected area and especially if you dry up any trapped water before storing it, that deterioration could take a very long time.
When neglected gunwales rot, it usually starts at the stems on the undersides of the deck plates and inwales, and where the machine screws securing thwarts and seat trusses penetrate the inwales. The thwart ends can rot too because moisture tends to be trapped between the thwarts and inwales for some time after use and the hole the machine screw goes through transmits the moisture into the wood.
I like to remove the thwarts and seat(s) at least some of the time I oil the rails. It lets you inspect the thwart ends and oil them (if not varnished) and get some oil into the holes. It also makes it easier to oil the inwales without getting a gunky mess or oil on the inside of the hull. If you do this enough, the oil residue inside the holes in the thwarts and seat hangers may make it difficult to get the machine screws through so you might want to enlarge them very slightly with a drill or rat tail file.
I have taken the rails off some old boats with weathered wood to inspect the hidden sides of the inwales and outwales. I found them to be pretty weathered but far from rotted. These days if I do that I will sand the inner aspects of the outwales and outer aspects of the inwales and seal them with epoxy rather than oiling that face, as I think it is more durable.
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