This was posted on another board some time ago. Good advice IMHO.
"Some interesting comments here regarding the use of engine oils and transmission fluids. Here is where I put on my smarty-pants and wax philosophical regarding lubricants.
Engine oils and ATF are composed of a large additive content, in the case of engine oils sometimes as much 60%. These additives are such things as detergents, dispersants, VI improvers, etc. Slick 50 and STP are pure additives, mostly just VI (viscosity index) improvers. Not only should these never touch your gun but also please do not put them in your automobile. They are nothing more than snake oil, no matter what your buddies tell you.
So what you say? Most of these additives have an at least slightly corrosive nature and over time will affect the finish of your gun. The additives are also designed to work in a very different and much more hostile environment than what a gun lubricant experiences and so are wasted (although we do not use enough lubricant for the cost to really be a concern). I do not recommend the use of industrial lubricants for firearms.
A gun oil is nearly identical to a turbine oil in that it has very little additive. Some have only an R & O (rust & oxidation) additive and some even have none at all. A lubricants job is to support two bearing surfaces and be able to handle the load enough to keep the surface defects (asperities) apart. It is this contact that produces wear. What happens in a gun, despite the high SAAMI pressures present in a chamber, during cycling is a relatively light load and so a light viscosity oil with very little or no additive is all that is required.
Any wear seen during the life of the gun will be due to two factors: the oil used being unable to support the load resulting in the breaking down of the boundary layer between the surfaces and more commonly, contaminants mixing with the oil and abrading the surface of the metal.
Many use Brian Enos grease or something similar. This is a perfectly acceptable practice as grease is simply an oil suspended in a wax matrix. The oil is released when pressure is applied and in fact the oil in the grease is no different than the oils such as FP-10 and Break-free and offers the same protection. The danger with waxes is the false sense of security because you believe it is doing a marvellous job while you have left it on your gun for weeks when all it is really doing is collecting contaminants and suspending them in the matrix thus accelerating the wear of your gun components. The wax will provide protection on a long shooting day without re-application but you must still clean your gun and re-apply for the next session.
In conclusion: Use a reputable oil or wax and clean your gun often. We have the luxury in the shooting sports of being able to not mind if our guns slobber a bit so use enough and wipe off the excess. On dusty days re-oil the slide halfway through the match.
Thus concludes today’s lesson and brief look into the kind of crap that clutters my mind. Thank you for listening.
Frank V"