Before using a lubricant, read the sections below that relate to the application you are using it for. Lubricants containing petroleum distillates (hydrocarbon solvents) or other flammable substances are not generally safe. They can end up in places where they won't evaporate before the gun is charged or fired. Combustion or explosion could result. WD40, many gun oils, cleaning solvents, silicone sprays and most similar products contain flammable chemicals. Products safe for firearms are not generally safe for airguns. Substances with any significant toxicity should not be used. Not only could this be harmful with regular contact but these can become aerosolized and inhaled while charging or firing. Which lubricants are safe is explained below. Section 1 covers low-pressure CO2 and self-contained pump models. Section 2 covers high-pressure break barrel, PCP and HPA guns.
SECTION 1 : CO2 & SELF-CONTAINED PUMP
Crosman suggests Pellgunoil which works great in warmer climates and comes in a convenient applicator tube. It's a little on the expensive side, especially if you do a lot of shooting. As a less expensive alternative any SAE 30 or 10W30 engine oil will work very well.
It's a myth you shouldn't use petroleum oil for airguns. This myth stems from the common misconception that petroleum distillates and oil are the same thing. They are not, oil is what's left after the distillates are removed. This myth is complicated even further because some distillates are commonly referred to as oil. The most common example is heating fuel which is often called heating oil. The oil that is safe is called mineral oil also known as heavy oil. Mineral oil is not toxic or flammable. It protects against wear and corrosion better than silicone or natural oils, will not harden and doesn't become rancid. This is why all engine oils are mineral oil. It will not hurt the seals in your gun. Modern airgun seals are made of materials like Nitrile and urethane which are specifically designed for use with petroleum oils and greases. I've been using engine oil in airguns for decades and have never seen an o-ring swell or dissolve.
It's a myth that engine oil is toxic, most are not. Some do contain potentially harmful additives, these will say so on the bottle and should be avoided. Used engine oil is more toxic and combustible, this is due to contaminants from fuel, combustion and wear.
It's a myth you shouldn't use detergent oil for an airgun. It actually works better than non-detergent. It has wear and corrosion inhibiting additives and helps remove moisture and contaminants as it works it's way through. An airgun doesn't experience the type of action or gain enough moisture for emulsion to be an issue.
Crosman Pellgunoil (Monolec GFS 8430) is SAE 30 engine oil with a high level of detergency. Because it is single viscosity it gets much thicker in cold weather. 10W30 works better when cold, especially in self-contained pump guns. In very cold weather 5W30 or even lower viscosity may work better.
The links below contain more information about Pellgunoil and other Crosman products.
CROSMAN PELLGUNOIL MSDS
LUBRICATION ENGINEERS INC AD FOR MONOLEC GFS 8430 STATING "EXCEPTIONAL DETERGENCY"
SELF-CONTAINED PUMP : These models should be assembled using engine oil to lube all the seals and moving parts. Add a drop of oil directly into the pump chamber every couple hundred shots. You can do this more often if the piston isn't working smooth or produces inconsistent performance. If you accidentally over-oil you can fire the gun with the barrel pointed straight up, unloaded and safely away from you. In that position excess oil settles near the valve stem and blows out through the barrel. To prevent oil-lock, only use one pump for each shot until excess oil is cleared from the pump chamber and valve. A drop of oil should be applied to all pivot points of the pump lever and one at the back of the piston seal. This will lubricate the contact points of the pump assembly. The owners manual should indicate other model-specific lubrication points.
CO2 : Like the pump models above, these should also be assembled using engine oil to lube all the seals and moving parts. The threads inside the front of the tube should also be oiled to keep them working smooth and prevent rust. For 22XX models powered by a 12 gram CO2 cartridge, a drop of oil should be placed on the tip of each cartridge before installation. This prevents the piercing pin from getting stuck in the cartridge causing the cartridge to get stuck in the tube. It also lubricates the valve, ports and barrel to preserve performance and reduce corrosion. After installing a cartridge the gun needs to be fired once (unloaded) to pierce it. This is not a malfunction, it's how these guns are designed to operate. Other types of CO2 guns will pierce the cartridge automatically during installation. These should have a drop of oil on the tip of each new cartridge as well. For bulk-fill models a drop of oil can be placed in the fill nipple before each fill. This is also safe if you operate it with air up to about 1200 PSI. Do not put any oil inside the fill nipple of dual-fuel PCP models. An explosion could occur during a rapid fill from an air tank. Refer to the PCP section below for more info.
SECTION 2 : PCP, HPA & BREAK BARREL
PCP & HPA : Without lubrication, PCP components can be difficult to assemble and o-rings are more easily damaged. Grease works better than oil for this application. It stays where needed to prevent corrosion and can be used to coat the inside of tubes more effectively. There is another reason grease is better. I had an incident where I was filling a 1701P with a scuba tank and the valve discharged a lot faster than I was expecting. It filled the gun to nearly 2000 PSI in just over a second. That was dangerously close to the fill rate needed to cause an explosion with oil inside the tube. Grease is generally more resistant to rapid compression autoignition than oil. This is often true even when the oil has a much higher flash point. Using grease leaves a more comfortable safety margin. If absolutely sure your gun will never be rapidly filled then most engine and silicone oils are perfectly safe. Otherwise, never use oil in any PCP gun.
For HPA tanks, a very thin coat of grease should be used on external threads and o-rings. This reduces wear, corrosion and makes removal easier. No oil or grease should be used on internal threads of paintball tanks. They are commonly filled at extremely fast rates, an explosion could result. Scuba tanks generally have grease on the internal threads to prevent corrosion. They are filled at slower rates so there is no risk of explosion.
It is critical to use the right lubricant for a specific application. It's not high pressure that causes these substances to detonate, it's rapid compression generated during a rapid fill. The fill rate required for rapid compression autoignition to occur varies by the type of lubricant and size of the container. A PCP tube capacity of 4.5 cubic inches would require a rate of about 2000 PSI per second for engine oil to autoignite. The rate required for larger tubes is higher. The rate for smaller tubes stays about the same. Based on this it is safe to establish 2000 PSI per second as the minimum fill rate required to cause autoignition of engine oil. I've done a lot of dieseling experiments with break barrel guns. I know from experience that silicone oil will combust as readily as engine oil when compressed at similar rates. Because of this I don't use oil inside PCP tubes.
A suitable grease should be low-toxicity, non-hardening, highly resistant to separation and relatively comfortable to work with. It should be easy to remove for maintenance and repairs. Most importantly it must be highly resistant to rapid compression autoignition. I test it by putting varying amounts inside the skirts of pellets and firing them from a high-powered break barrel gun. I use a chrony to determine if velocity increases from extra pressure generated by dieseling. Even in the absence of extra noise and smoke, slight dieseling can be detected if the pellets tend to fly a little faster. If no increase in noise or velocity is noticed the grease is safe. All greases will diesel under the right conditions, the point is to find one that is highly resistant to dieseling in this application.
High-temp wheel bearing grease with a high flash point is the type that works best. Some are more toxic and/or not very pleasant to work with. Most are quite resistant to dieseling but they need to be tested to make sure. The product I sell as AIRPOWER™ HIGH-PERFORMANCE PCP GREASE was the best one I tested out of dozens in this category. It's very low toxicity, comfortable to work with and highly resistant to dieseling.
There are other types of grease that are safe to use but those tend to fall short in other ways. Krytox is one, it's extremely safe. It's also extremely expensive and doesn't prevent corrosion as well as wheel bearing grease.
Silicone grease is a poor lubricant for mechanical purposes. It's not used for most types of machinery because it tends to harden and does little to prevent wear or corrosion. Dielectric versions resist hardening and work great on electrical connectors but don't resist dieseling very well. Silicone brake and scuba greases failed in the dieseling category also.
Natural greases are not very good for long-term lubrication and corrosion prevention. The problem is they tend to harden, separate, turn rancid and diesel easily. My Omega compressor operates straight from the factory with one of the best natural greases in the industry. That grease diesels so well it added over 200 FPS when tested in a break barrel. It also separates, all the containers of it that I have purchased were leaking oil and had droplets of it inside. The compressor has a slow-moving piston so it's safe in that application but I would never use it in a PCP tube.
I tested a variety of hi-temp lubricants for combustibility. I was not able to ignite several engine and silicone oils with a torch but both dieseled well in a break barrel chamber. The Omega compressor grease (Renewable Lubricants NLGI #0) was more difficult to ignite than any other grease I tested. As described above it dieseled very well. The reason for this is a more liquid version of a substance tends to diesel easier than a thicker version. This is true even when the thinner version has a higher flash point and is more difficult to ignite with flame. I tested the NLGI #2 version of the same Renewable Lubricants grease and it dieseled a lot less. NLGI #2 grease is thicker than #0. All of the wheel bearing greases I tested were NLGI #2 and most of them would not diesel. With the few that did it was very slight. Silicone greases tend to be thinner which is why they tend to diesel.
BREAK BARREL : One drop of oil can be added as often as every couple hundred shots. Many guns will not require lubrication this often. The owners manual should give you a rough idea of how often is necessary.
Some manufacturers insist that only silicone oil be used. The reason for this, it is widely believed silicone oil doesn't diesel and/or petroleum oil will damage the seals. Neither is even remotely true. Some models have a leather piston seal that requires petroleum oil. Most have seals made of urethane or other synthetics. These synthetics work extremely well with engine oils but don't last as long or perform as well with silicone. It is widely known in most mechanical fields that petroleum oils lubricate better, prevent wear and corrosion better and are less harmful to you and your machinery when combusted. Silicone oil diesels in a break barrel gun just like engine oil does. After a few shots the thinner components of the oil burn up. It's how well the residual oil works from that point on that is important. Silicone oils work relatively well initially but engine oils work better, especially over time. Most guns will last longer and shoot better if you use engine oil.
After adding a drop of Crosman Silicone Chamber Oil (RMCOIL) it tends to diesel for a few shots before it settles to a relatively consistent performance. Engine oil tends to burn off a little quicker and produces better consistency with higher FPS.
In February of 2017 I tested a new Crosman .177 Steel Eagle right out of the box. The first shot with a 6.9 grain pellet was 1483 FPS and dieseled heavily. That was with only the original factory oil in the chamber. The second shot was 1428 FPS again with heavy dieseling. It took a total of about 10 shots for the dieseling to stop completely. After that it produced a ten shot string averaging 1031 FPS. The high was 1067 and the low was 997.
I added one drop of Mobil 1 10W30 directly into the compression chamber. The first shot with the same 6.9 grain pellet was 1445 FPS with heavy dieseling. The second shot dropped to 1231 with mild dieseling. The third shot didn't sound like it dieseled at all but the FPS was still elevated at 1117. The fourth shot was 1087 and every other shot after that was between 1071 and 1085. The gun was more consistent, shot smoother and produced more power than it did originally.
If not done correctly, dieseling your break barrel can cause damage to the gun and personal injury. Using a flammable substance can cause fire or explosion. Adding too much oil to the chamber or oiling too often can damage the piston seal and cause combustion residue to build up. Excess oil can work it's way around seals and combust outside the compression chamber where damage to the gun and personal injury could result. The method I describe above for testing grease by putting it in the back of pellets is relatively safe for those who fully understand what they are doing. There is no completely risk-free method.