Oscar Gaddy had in his article about re-rust blue and etching of barrels, he suggested motor oil or linseed oil.
I have found out when I re-rust blue/etch that the boiled linseed oil put on very thin with a cottom ball is a very good way. The linseed oil hardens and if you are careful enough to put it on thin, it will look great and stay that way.
I have used the motor oil too, but I had places where some very light rust has formed, but this was with barrels just re-rusted. Now if you are only worried about keeping your barrels free of rust, I'm sure the other methods work good, but to me the boiled inseed oil really protects it and gives it a nice shine.
