Remoil. Spray it on, spread it around with a cloth, don't worry about it for months.

I used to use mineral oil, which is terrible. What makes Remoil great is that it doesn't dry out in the air (leave a remoil soaked cloth out and it will still be "wet" a week later...try that with mineral oil) and it's not easy to wipe off. I've used it on my group's trianing blunts 3 months ago and they haven't rusted since.