The NAPA house brand penetrating oil in the spray can is pretty good stuff. I'll go out on a limb and say it is as good or better than Kroil, WD40 or whatever, and, pretty much everybody has a NAPA store somewhere in their time zone, anyway.
The spray version of Rem-oil, and Birchwood Casey Sheath work OK, and I usually have them on my bench. I used up the last of a spray can of Breakfree, and need to replace it, good stuff, seems about like penetrating oil with a bit better lube qualities.
WD40 gums up far faster and worst in the presence of mineral oil then it does by itself. The "lights" or carrier in WD40 are designed to evaporate quickly, and they will happily take the liquid component of any mineral oil with them, leaving a nasty varnish behind. Clean all traces of oil off before you use it as a rust preventer. You will be happier with the results.
Best,
Ted
