"Have you ever wondered why we're using oil on these unsealed machines we call guns? You won't find that in any other industry - unsealed, sliding machines get grease."
~Cherry
Educate yourself - Introduction
Have you ever gone under your car, and noticed that any oil leak will be filthy, all the way through, yet if you pinch off a blob of old grease near a fitting, it's pristine and clean underneath that crust?
This is because oils are designed to attract and suspend friction contaminants, and to flow - generally in sealed systems like high-speed bearings or more complex machines with filters, pumps, and reservoirs. Oil flow is used to both transport friction contaminants away from friction surfaces, and to separate moving parts on a fluid film.
Flow is the primary reason why it is so common to clean and oil a gun, only to find after a short period in storage the oil has nearly disappeared. Typically, it spreads and thins out, often evaporating. It is also why we have to relubricate firearms much more often with oils during use - it flows away from friction surfaces with time, gravity, and cycling. Critically, with oils being used to suspend and flow friction contaminants, this can happen in your firearm - sand and fouling, instead of remaining outside of friction surfaces, can be suspended and migrated right into them.
Greases, on the other hand, are designed to stay put, and trap friction contaminants on contact, keeping them out of friction surfaces. Because of this, you will almost never find unsealed, sliding systems using oil outside of the gun world - they use grease. There are some exceptions, but understanding this basic difference in the nature of lubricants is critical.
Oils are designed to suspend contaminant and flow - greases trap contaminant out, and stay put. Know the best use of each.
This difference between oils and greases is also a core concept in tribology, the science of friction, lubrication, and moving surfaces - it applies broadly, regardless of the type of machine in discussion.
Another key piece of information is that greases are not what most of us commonly assume - it's best to think of them like a sponge holding oil, squishing it out under force and then sucking it back up after the work is done. But it's the oil that lubricates. Automotive oils and greases, for example, often use the same base-stock of oil - this difference is that greases suspend this oil in microscopically fibrous 'soaps' or 'thickening' complexes, very similar in concept to that sponge. Modern greases can be engineered in a host of thicknesses, ranging from the hardness of cheddar cheese, to the lightness of cooking oil.
Under a microcsope, greases look and act more like a sponge - oils suspended in a fibrous complex that get released and loosen up with work.
Most of our misconceptions about grease come from things like cosmoline - which is a waxy preservative, not a lubricant - or experience with industrial or automotive greases, which are generally a NLGI #2 thickness and require far more energy to drop their oil out efficiently than firearms are designed to operate under. However, lighter greases can be engineered to not only trap friction contaminants out of your gun's friction surfaces, but to stay put for months and lubricate for thousands of cycles in your weapon's action, under the harshest of conditions.
Given that oils suspend contaminant and flow, while greases trap contaminant and stay put, at this point a reasonable person might ask, If this is all true, then why do we use 'gun oils'?
The thing is, we didn't always, at least not as primary lubricant. World War II was won with guns lubricated with grease. We go into this in the History section.
Historically, however, with the exception of a few gross examples, the biggest challenges to reliability in semi- and full-auto firearms hasn't been the firearms - it has been the lubrication technology available, and operator knowledge. Unfortunately, much of what is common lubrication knowledge in the industrial world to mechanics, millwrights, and even farmers, is either unknown in the shooting community, or contaminated with myth and misunderstanding.