Since you have a TF99, I guess the best thing is to cover spring guns for now.
Dieseling refers to the ignition of any combustible substance in the compression chamber at the time of firing. When a spring gun is fired, the piston is driven forward by the spring, and the seal compresses the air in the tube. This only takes a few milliseconds. Now, when a gas is compressed rapidly, a by-product of that compression is heat. The temperature inside the chamber is for a couple milliseconds raised well above the flash point of any petroleum based oil. So, any oil in the chamber burns in a tiny explosion. Its basically the same principal as a diesel motor. This tiny explosion raises muzzle velocity well above what it should normally be, sometimes by several hundred feet per second. Its quite possible to get a .22 pellet to break the sound barrier. OK, so it makes lots of power, but the problem is it destroys the gun if allowed to continue intentionally. Not only is the pellet driven by the explosion, but the piston is forced back violently against the spring, which is still trying to go the other way. Dieseling leads to short spring life, spring breakage, burnt seals, and can actually break the gun. If the gun is cheaply made (and maybe if it isn't), it is entirely conceivable that using detonation to intentionally increase a guns power can over time cause the gun to come apart, causing injury to the shooter. There was a story some time ago from the UK that someone had been experimenting with powering a spring gun by intentional detonation from various sources such as oil, ether, etc, and after a while the breech plug separated from the gun, causing the spring to fly out and the whole thing- spring, plug, and whatever else struck the guy in the face, causing severe facial injuries.
Dieseling when a gun is new or after its been serviced is not a problem and is expected. New guns generally have some oil in them from the factory to keep rust from setting in during storage, and maybe a bit of residue from production. New guns sometimes take a hundred or so shots to settle down, sometimes only a few.
Repaired guns usually have a bit of oil in the chamber from having the piston removed. When the piston is put back in, the seal scrapes the cylinder wall, and picks up small amounts of oil, pushing it like a squeegee into the chamber. This usually burns off in a few shots. Generally, using a heavier pellet will accelerate the process of burning off the oil. In either case, you'll notice a bit of smoke when you open the gun.
All that being said, there are certain guns that are designed to use detonation as a power source, but the 99 is not one of those.
Just about any spring gun uses a bit of detonation for power. A good seal scavenges microscopic amounts of oil from the cylinder wall, which is consumed in the firing process. Thats normal, and in most guns it is normal to see a very light haze in the barrel after firing. The problems occur when the oil levels get to the point where combustion is the dominant force driving the pellet, not the expelled air. Thats when you get loud reports, smoke (and in extreme slick50 cases) sparks or flames from the muzzle.
Slick 50 treatment is basically intentionally using detonation by spraying Slick50 into gun. There are a few who advocate this practice, especially with leather seal guns. The Slick50 saturates the leather, and burns off a little at a time, so the detonation lasts for quite a number of shots. This isn't recommended, as it can damage the gun.
Spring tar is just that- a black very sticky substance that is put on springs to dampen vibration. A light covering is put on the outside of the spring before it is installed in the gun.
Moly is a lubricating grease, molybdenum disulfide. Its used inside the gun in small amounts to lubricate the outer piston walls and any other moving metal-to-metal contact. Again, its applied to the piston and whatever moving parts require it before assembly.
Seal life depends on the gun, the user, and what its made of. Cheap guns have cheap seals. I've heard so many stories about Chinese guns that burn up seals in under 100 shots, seals that split, crack, come apart or what ever. Quality guns usually have good quality seals. Some last thousands of shots. Older guns like the early Dianas have leather seals that last virtually forever. Many of these guns that are 50+ years old are still working fine with the original factory seal. The main reason that leather isn't still used today on a lot of higher end guns is that it doesn't make the power of a synthetic seal.
Lots of heavy dieseling can ruin a seal. Synthetic seals are especially prone, as they can burn or crack from thermal degradation.
Guns with synthetic seals require little maintainance. Some will say you need to add cylinder lube every so often, but synthetic seals don't need that. Leather seals do need to be lubed from time to time or they will dry out. Use a good heavy silicone based oil, 2 or 3 drops in the chamber port is good enough.
Barrel cleaning is another important subject. Airguns generally don't need it unless they have been dieseling heavily, in which case there will be carbon deposits in the bore. There's no caustic powder residue as from a firearm, and leading is mostly minimal. If the gun is shooting as it is intended, it should take many thousands of shots before the barrel needs cleaning. Many target shooters don't ever clean, as it can change the accuracy or consistency of a gun. Some have not cleaned their barrels in years. Cleaning can do damage to the rifling also. Airguns generally have fine shallow rifling, and it doesn't take a lot to screw it up. Just one time with a wire brush, or a metal cleaning rod can do it. Using nylon cord or weedwhacker line as a pull through can be bad too. This can lead to damage to the barrels crown if the line is allowed to rub on the end of the barrel. Any grime carried out on the cord will slowly grind away at the crown, and eventually accuracy goes away.
Using fire arms type cleaning agents is also to be avoided. These usually are some sort of solvent that will attack most petroleum-based plastic or rubber products. Since most seals in the gun are some sort of plastic or rubber, this is bad and will greatly decrease the life of the seals.
