I just use cheap old mineral oil or coconut oil and have never had a problem. They're both food safe too, unlike some of these other options. Coconut oil is a solid at room temperature, but if you rub it between your fingertips a bit (raise the temperature some) it'll turn into a liquid. Easy to apply, gets into uneven surfaces great, and hardens almost like a wax once you stop warming it with friction. Not recommended for very warm climates though, and it will go bad if left for more than a few years. Under those conditions I wipe down with mineral oil.
Also be careful storing in leather sheaths for long periods, at least don't store them jammed down in the sheath like you would carry them around. Leather releases moisture over time, and without protective oil/wax just having carbon steels in a leather sheath that can't breathe well will cause rust problems.