All I use is F-ONE Oil, ever since hearing Roger Sadowsky talk about it, in a setup video. It’s for fretboards.
For the back of the neck, I believe Lakland (I know Carl uses it) uses Tru Oil thinned out with mineral spirits, on the US necks. I’ve yet to reapply Tru Oil to any Lakland necks. But, I’ve sanded off all the finish on many non-Lakland necks and applied 100% Tru Oil (several layers) to them. It can leave a thick glossy finish, a thick satin style finish, or a thin oiled feel finish, dependent on how you apply and how much you sand in between coats and after the final.