"Greases ain't your momma's petroleum jelly. Think of 'em like a sponge, holding an oil and additives, that automatically release that oil to lubricate when hit with a certain amount of energy, and suck it back up when the work's done. The engineering behind modern greases can be more difficult than some advanced oils."
Educate yourself - Greases
Modern greases are surprisingly interesting, and tremendously effective - some involve much more complicated engineering than oils. The most important thing to know is that grease is far more than it appears.
Perhaps the most surprising thing for most folks to learn is that it’s actually not analogous to a single ‘thing’ serving as a lubricant, as may be understandably thought of when people consider grease-like items they may be familiar with, such as petroleum jelly or bacon grease. Modern greases are an engineered matrix of carefully selected components, and are actually most analogous to a sponge, suspending both an oil and a properly selected additive package. Deceptively, what you actually see in a modern grease is about 80-90% oil, depending on the particular grease.
Modern greases have two main components: the sponge part, which are known as 'thickeners' or 'soaps', and the oils that are soaked up inside it. These thickeners, which can be based on lithium, clay, calcium, aluminum, and others, are chosen for their specific properties. Some withstand pressure well, others are great at withstanding intense heat, and yet others are better at resisting sheer forces or water wash-out.
Modern greases are a multi-component matrix of materials, acting more like a sponge, suspending oil and various additives. Seen here is highly magnified lithium complex
The oils that are suspended in these thickeners are equally specific in being chosen for the task and machine at hand. A substantial portion of this selection process will involve understanding the operating dynamics and operating environment of the machine. Factors such as speed, direction of movement, load, and temperatures at both start-up and operating speeds are all taken into account. This determines oil type and viscosity, as well as additives that will be selected for the grease. However, a tribologist also has to be intimately aware of how different oils, thickeners, and additives react to each other, as the wrong combination can cause hidden problems that may not emerge in laboratory testing, but only under the longer-term realities of use in the field, especially in exposure to the elements and contaminants.
Greases drop their oil when worked, loosening up, and solidify back up when work stops - this is "thixotropic"
To understand how they work in action, think back to that sponge analogy. Any given grease has been engineered to need a fairly specific level of force or energy applied to it for the oil to essentially get 'squished' out of suspension. The more thickener there is, the more energy is needed to efficiently drop that oil out of suspension. When this happens, a modern grease will appear to 'loosen up' as the oil gets deployed - and it is the oil that does the lubrication, although the viscosity provided by the thickener can help enhance the ability for higher speed parts to get hydrodynamic lift. Interestingly, depending on the specific grease, once the work stops, to varying degrees the oil gets ‘sucked back up’ into the thickener again. So while greases may look something like a solid at rest, they loosen up as soon as energy is applied, and then solidify back up when it's done. The ten-dollar word for this back-and-forth change in physical viscosity is “thixotropic”, and it is part of how greases 'stay put' so effectively, how they last so long under work forces, and importantly, why you often need so little of it.
The biggest visible differences between greases are consistency – overall weight and firmness. The National Lubricating Grease Institute - NLGI - ranks greases on a 9 point scale of firmness, ranging from #000 to #6, with #6 being as hard as a block of cheddar cheese, and #000 being the consistency of cooking oil. Because of the nature of grease - being 80-90% oil, suspended in a thickener - the lighter the grease, the more oil it has. Some of the benefits to grease are that they typically are much easier to work with, don't spill easily, and clean up well compared to oil, and they're very forgiving of accidents. However, one of its few annoyances is that in some situations a little of the oil may bleed out during storage, and the lighter the grease the easier it is for this to occur. While only needing to be stirred to put the oil back into suspension, it can be surprising to encounter, but it's also important to remember that the oil you do see in bleed is a tiny fraction of the overall contents, and is easily remedied.
The most common grade to encounter is #2, which are what the vast majority of everyday automotive and industrial greases are. It's around the consistency of peanut butter, and it is also a big part of the reason most shooters assume 'grease' is too thick for their guns - and it is. While a #2 grease will lubricate a firearm to a certain degree, it contains far too much thickener to do so efficiently, and that thickness actually absorbs some of the energy the gun is supposed to be cycling under. Additionally, many #2 greases are automotive greases, which often have higher amounts of what are known as 'tackifiers' - components that make a grease sticky. You generally do not want a thick, sticky grease in your guns. As with oil, while it can work, #2 greases generally narrow the window between friction and energy a gun operates under, leaving less room for error. The degree depends on the specifics of the gun.
The key thing to remember is that #2 greases are used on cars and industrial equipment, and are engineered and optimized for tons of pressure being applied. Bearings in cars and tractors are under vastly more pounds of pressure per square inch than guns, and greases designed for this type of torture simply require far more energy to be applied to efficiently drop their oil out of
M1 Garand grease - a calcium based grease, issued in cups the diameter of a dime. Grease goes a long way.
suspension than guns are designed to cycle themselves under. It can work, especially in helping prevent some shock loading, but reliability is largely dependent on the particular type of gun. However, for those using #2 greases on guns, they are getting some key advantages over oils - the biggest of which is simply exceptional endurance, the ability to help trap friction contaminants away from friction surfaces, and generally much easier clean-up. Plus, like using motor oil or Automatic Transmission Fluid, it can be purchased in lifetime sizes for just a few bucks. Just know they are not optimized for guns, and calculate your risks.
Another final item worth noting, should you be in the market for industrial greases, are colors. Greases come in several common colors, including white, tan, blue, black, and red, which companies tend to group by application purposes, for easy visual identification. Generally, blackish greases tend to be very heavily fortified with Molybdenum Disulfide as a boundary lubricant for high loads, which is where they get their color. And it's not uncommon for white greases to have higher levels of zinc additives, which, as anti-oxidants, are excellent corrosion inhibitors. Red greases, especially red lithium-complex greases, are some of the most common, with literally hundreds of different formulations being marketed in automotive shops and in industrial supply. While it's not an absolute, companies often color their lithium complex greases red to associate it with the excellent high-heat endurance properties of lithium complex, but lithium complex greases can also be found in tan, blue, or yellow as well, with zero meaning behind it. The key thing to know is that color is somewhat arbitrary and does not dictate use across the industry, and instead are generalities, with companies choosing within their own lineup of offerings a specific set of colors to help with quick visual identification.