So you've got a 98 custom. Good choice. They're reliable, accurate, (with a new barrel, I recommend you upgrade) easy to maintain, and are built like a tank. However even tanks need cleaning every once in a while.
Disassembly
Please do this on a towel, or open area where you won't lose any parts.
Most of the parts will be greasy, so don't do it on your mom's carpet.
The first thing you need to do is unscrew your barrel.
Next, you need to unscrew the six screws on the left half of your marker with the allen wrench that came with your gun.
This picture is from the 98 custom instruction book. I modified it. They get credit.
The screw in red should not be unscrewed. It is for velocity adjustment, and it doesnt hinder taking the gun apart.
Also, loosen the screws under the air source adapter. (it's the thing under the back handle) You don't need to completely take them out.
After all the required screws are out, the reciever halfs should come apart. Leave the right half on your work surface, and lift the left half off.
You should now have a good view of all your internals.
Cleaning
If your marker has been previously used, there might be paint from a chopped ball in the front bolt. A chopped ball is when a paintball only feeds halfway, and the front bolt chops it in half.
Remove the front bolt, and wipe it off with an old rag. It should be pretty dirty, and greasy. Don't worry about taking the old oil off of it, we'll get to that later.
There also might be paint or grease in your barrel. To get rid of that, just run the whole barrel under warm water until all the paint and grease is gone, and leave it out to dry.
If there is any visible dirt inside the body, wipe it off with a damp rag. Don't worry about rust on the body, Tippmann's are made of aluminum. The only things that will rust are the linkage arm, rear bolt, screws, and springs.
That's pretty much all for cleaning, if there's anything weird in your gun, a leaf, a rat, or cat hair, remove them. They don't belong there.
Oiling
After everything is nice and clean, you'll need to re-apply oil. This should be done about every 2 months if you haven't used your gun, and after every use at a field.
The gun should have come with some oil, but if it didn't, pick up some Hoppe's Lubricating Oil.
A drop or two on all mentioned items should do the trick.
Also taken from the 98 custom owner's manual, and modified by me. They get credit.
Oil your front bolt's o-ring, shown as "A" on diagram.
Next, oil the linkage arm. It's "B" on the diagram.
After that, oil the rear bolt's o-ring. It's "C" on the diagram.
Don't forget to oil the buffer o-ring. That keeps the back bolt from smashing into the end cap. It's labled "D" on the diagram.
Re-assembling
Now you're almost all oiled up. However, there is one more thing you need to oil. The gun must be assembled for you do do it.
To re-assemble the gun, simply make sure all the parts are in their proper place, and place the left shell on top of the right one. Then, replace the rest of the screws. The longest screw goes on the front grip. Smaller ones, doesn't matter which holes you put them in.
Tighten all the screws, including the ones at the bottom of the Air Source Adapter.
Almost done. The last thing to oil is the Air Source Adapter. Labeled "E" in the diagram below.
Taken from 98 custom instruction book, and modified by me. They get credit.
There will be a tiny hole. (on the right side if you're looking straight into it)
Put 2-3 drops of oil into that tiny hole.
After that, screw in your full CO2 tank, and dry fire it WITHOUT the barrel 10-20 times.
This will distribute oil evenly through the pneumatics.
The reason you want to do it without the barrel is that oil will get all over it, and you don't want oil over the barrel you just cleaned. It won't hurt it though.
Take off your co2 tank, and you're all done.
Good job, you've just cleaned and oiled your 98 custom. Doing this will ensure that your gun has the longest, and happiest life possible.
It will also hopefully cut down on costly repairs related to not taking care of your gun.
If I forgot anything, or got anything wrong, please tell me so I can change it.