I use and have used WD-40 on every single razor and have for 3+ years now... ZERO rust issues, and yes there are over 100 razors in the bathroom.... Some of these razors haven't been used in over 1 year, no problems...Zero issues with reaction to scales either...(I would not use any oil with un-sealed wood)
Now I do admit to a rather strange ritual before putting a WD-40 coated razor on my face for shaving,,, I WIPE IT OFF, then strop, I then also rinse the razor in hot water before shaving with it, I like them warm... I guess that isn't done with all these other oils you guys just slop that stuff all over your strops right????
If you guys actually take the time to read the tests that have been done about rust prevention that are easily found by Google you will find IIRC 2-3 products that score better than WD-40 in the tests, they are gun oils...
BTW mixing any oil with dust, dirt, and powder blow back, inside of a firearms working parts will gum it up...This is why no wet lube is reccomended for that application...
I have absolutely no clue as to why the WD-40 is staining the op's razor, but my guess, is it has nothing to do with the product, who knows though, without doing some tests, that is all just guessing...or WAG actually...
Now also, I never ever "Spray" a razor, I spray a piece of TP and gently wipe the razors steel...
PS just the first test I found, there are many many more make sure to note the part about not using it inside of gun parts...
http://www.thegunzone.com/rust.html