“Hey sensei, what should I be using on my shinken?” says one of my students.
Steel is steel, so anything that protects steel should protect a sword. Having said that, bluing or browning is likely not a good thing to put on a sword….
Then you get into “what’s SUPPOSED to be used” on a Japanese sword.
I’m not a collector and don’t expect the sword I use for iai to be around in 300 years so I don’t actually care much what goes on it, as long as the rust is kept off. I have been known to use Burke’s Gun Oil which is made by a student of iai in this area, we’ve got a bottle of the stuff in the dojo for anyone to use.
The things you want on your sword steel are those that are basic (as in simple and as in non-acidic), and that protect the steel from rust. You also want things that will not stain since we all like our shiny blades, that’s why bluing or browning is out.
One thing nobody ever talks about is “sword water” for cleaning their blades. If anyone is interested, it’s water that has been filtered through hardwood ash and it’s used to take all that nice water soluble stuff off that the oil and uchiko won’t.
Wood ash and water means lye of course, potassium hydroxide, which doesn’t leave a salt when it dries like sodium hydroxide, especially at the concentrations needed, but will neutralize any skin acids you put on the blade. Sword polishers use washing soda or similar in the water for the same reason, to prevent acid and rust. Don’t use washing soda, it dries to a nice white salty mess.
All those horrible tameshigiri scuff marks on the sword that uchiko and oil won’t take off? In our box we’ve got some rubbing alcohol for those who cut their fingers instead of their targets and that works a treat for scuffs. It dries off fast (don’t pour it into your tsuka) and then you re-oil.
Bent sword? Put it over your knee, it’s what the polisher will do… it’s steel.