Oh I see what you are saying - you're talking about oiling the fingerboard, not the back in the neck. Before answering that question - I want to clarify, does your bass have a polyester coating over the ebony, or is bare wood? Tung oil won't actually create a harder surface to play - it's a penetrating oil that will absorb into bare wood. If you try to put tung oil over a sealed finish, it will just kind of puddle on it, and slowly turn into a sticky mess. You don't want to put tung oil over a hard wood finish. Another issue with tung oil (and many finishes, for that matter) is that there are often a lot of byproducts and other gunk in stuff commonly sold as "tung oil" or "lemon oil." Hard to know what sure what you are getting. For treating natural wood fingerboards I suggest "F-0ne" Fingerboard Oil that is common in music stores and available on Amazon.