lungimsam,
I think some/many of us are a little confused by your terminology. You don't generally see rosewood or ebony fingerboards getting "sealed," if by that you mean lacquered or finished some other way. That's not to say they don't exist, but if they do, that's pretty rare; and I don't think Guild did that. Rosewood and ebony fingerboards are left unfinished, with the possible exception of the edges in an unbound fingerboard.
You do, however, see finished maple fingerboards on any number of electrics out there and even quite a few acoustics (not Guilds, that I know of).
Anyway, just to be clear, the linseed oil / furniture polish (lemon oil) discussions above are what most people would call "treating" or "oiling" the fretboard.
It wasn't clear to me re-reading your original post what you were actually referring to. As far as treating the fretboard is concerned, it's really a matter of taste. Rosewood will darken considerably when you oil it. Oil gets drawn into the rosewood very quickly and you can see the oil dry out. Ebony is much denser and, in my experience, barely needs any oil at all. Regardless, you use it sparingly. I use bore oil (a product for woodwinds) on my fretboards and bridges. I've had a small bottle of this stuff for over 10 years, and I've barely used a third of it. Boiled linseed is good too. I've heard people swear by Fret Doctor. Anyway, the key, as with guitar polish in general, is to avoid anything with silicone in it. I would not use a furniture polish (lemon oil) unless you're certain it has no silicone.