The only thing I ever heard about WD40 was years ago from a chemical cleaner salesman. He said the stuff was originally concocted for use as an industrial solvent and that it works great for cleaning road tar off your car. I don't know about the former but he was spot on with the latter. I've used it for displacing water when cleaning up black powder firearms. It does work as a penetrant but I've come across other products that work better.
On a somewhat related topic I've found that ArmorAll makes a great fly line dressing and flotant for dry flies!
