We recently had a long discussion about this on another forum. The experts said not to use standard mineral-based commercial bore oils. Any non-drying plant-based oil that does not leave a sticky residue is OK. Almond oil should be OK, but tea tree does leave a nasty residue and I would avoid it personally. I use pure camellia oil, which is the oil of choice for shakuhachi flutes, and has been used for that purpose for centuries in Japan. Jojoba oil is supposed to be very good as it is light, but be sure to get it without PABA or other additives.
Oil can be applied liberally to the bore, with the caveat that care should be taken that it does not run into tone holes and get on the pads. I take a flute swab stick, wrap with a cotton cloth (long enough that I can hold it tight at the end), apply a fair amount of oil around the top of the cloth and "paint" the entire inside of the bore. You'll need a thinner strip of cloth at the top of the bore.
I also lightly oil the outside, avoiding the keys, but this is not so important. I oil repeatedly until the oil is not absorbed overnight, then swab the excess out.
At that point I will probably do a thin coat every couple of months, following the same procedure.
The name of the game is to replace the missing natural oils in the wood, so that the lignin structure does not absorb moisture. As water is absorbed the wood swells, and when it evaporates the wood shrinks, and it is thought that this cycle over the years can cause or contribute to cracks. It also limits saliva damage to the wood (saliva has compounds that break down organic materials), and prevents calcium deposition in the bore.
The Romamian instrument I have is not a tight-grained hardwood and would probably absorb a huge amount of oil. With that one I would probably cost twice and recoat once every couple of months, but I would not try to saturate it, I think.