I'm refinishing a couple of Springfield Krag rifles, and I'm wondering what type of oil would work best to give the jet black finish the various parts had. I tried 10W-40 and 5W-30 (as that is all I had around), and after the second try I start getting a brown scum on the parts along with some other colors. I'm assuming this is because of the additives. Any advice would be greatly appreciated. Thanks in advance.

Sorry I should have said "Oil-Blackening" instead.

There is this:

http://www.thegunshop.co.uk/metal_blueing.htm

Paul