I've heard many answers to this. I've had my workshop for 50 years with numerous cast iron surfaces including metal lathes and milling machines with no problems. Of course it depends on the moisture and salt content of the air where you live. I live in moisture-laden Atlanta, Georgia USA where you have to cut your way through the humidity in the summer. In all of these years I have used WD-40 which was developed for the U.S. Navy to deal with rust on machine surfaces and I have no problem if it is done properly. WD-40 is made to DISPLACE moisture on metal surfaces. It enters the pores of a metal surface and forces the moisture out, then forms a coating to keep moisture out (I read this on their website). About once a year I liberally spray WD-40 on my cast iron machine surfaces and tools and leave it overnight to do its job. Then I wipe off the surfaces the following day with a paper towel. It leaves no noticeable oily residue, does not soak up into sawdust, and doesn't get on any wood I work with. No rust in my shop! I'm sold on the stuff.
Planeman