Okay folks. I’ve got some experience with this topic in particular as well as a background in science. Please:
1. DO NOT USE MOTOR OIL, TRANSMISSION FLUID, ETC. for pocket knives, guns, etc. Sure, it can work. So can a lot of things. These particular products are not made for the intended purpose. They are not good for you. Why put toxic stuff on or in an intimate EDC item that you carry and use?
2. DO NOT USE WD40 AS A LUBRICANT OR RUST PROTECTANT. Again, it can work. It does penetrate well. However, its efficacy as a lubricant or rust protectant is relatively poor. It is definitely toxic.
A word on toxins: yes, dose matters. No, a drop of these things won’t kill you. However, they are CERTAINLY not good for you. They can be biologically persistent. Taken in context with all the other toxic stuff you get exposed to, they can contribute to poor health in a cumulative fashion. As a good friend of mine likes to put it: “why lean into the punch?”
Unless you are stuck in a jungle somewhere or living in a third world country, you have plenty of better options. Several products mentioned in this thread will work MUCH better. Some are still toxic but that depends on the product. I wasn’t the first to recommend a high-quality and non-toxic option. Whether good ol’ fashioned Ballistol or the excellent Lubriplate FMO 350-AW; do yourself and your knives a favor!