Another shooter and I got into a discussion of grease vs. oil several years ago. I use grease but the other guy argued that oil does a great job in engines and their parts are subject to a lot more heat, dirt and pressure than those of firearms. His point was correct but engines have a pump that continually replenishes the oil film on the moving parts; guns do not, so when that film wears away, there is no new coating to take its place.
Ed