Wow! I never heard any of those things about WD40! I know that WD stands for water displacement, and that it was invented around WWII. It is a silicon lubricant with an evaporative carrier, but I have never seen it discolor metal. 3 in 1 oil is good stuff, but it is also a penetrating oil for rusted bolts,ect. I have seen that remove bluing that was not completely cured, as bluing is a form of rust. Because of that I use good old 30 wt. non detergent motor oil to cure bluing & browning, as well as on my mill and lathe.

 

Greg