This is a question that we get a lot, so we wanted to be sure to address it fully. It seems that there is a ton of misinformation out there about why WD-40 is bad for your knife blade, the idea being that it will accelerate the rusting process as opposed to prevent it. We could find no real evidence for this, and in fact the whole theory seems to have been debunked. While it is true that WD-40 has petroleum in it, there is nothing about its composition that will cause the blade to rust any faster than a different lubricating oil.
HOWEVER, this doesn’t mean WD-40 is the best option for keeping your knife clean and lubricated. WD-40 was not designed to lubricate or protect; it was actually designed to displace water from mechanical components. For this reason, it is not nearly as effective for use with knife blades than more specialized oils, like Breakfree CLP or EEZOX.