I hope this helps you.
In regards to changing the oil it is more involved than just draining the oil tank under the headlight. Here are the steps you will want to take.
1) Find the hose that runs from the oil tank to the motor. Its on the left side as you sit on the quad. Have an oil pan handy and a big roll of paper towels (your going to need themhttp://forums.atvconnection.com/images/smilies/smile.gif) Loosen the hose at the motor and quickly place it over an oil pan and let drain. This will drain the oil tank.
2) The same hose you removed at the motor you want to remove underneath the oil tank. THere is a mesh oil screen in there that should be checked from time to time. Since you have no way of knowing whether the prior owner cleaned the screen I highly recommend checking it.
2) Remove the oil plug underneath the motor. Let drain into oil pan.
3) Remove oil filter. THis is where you are going to need a lot of paper towels.
4) Your almost there... Put in a new oil filter, replace the drain plug on the bottom and tighten the oil line between the tank and the motor back up.
5) Place ~3qt of oil into oil tank and start her up and let it run for a few minutes. Make sure the red light on the dash goes out within a few seconds of startup or else shut it down. Let sit and check oil level. My DS took about 3.3qt everytime I changed the oil. Use a good 10-40wt conventional or "motorcycle" synthetic that is safe for wet clutches. If you use an automotive synethic you clutch will end up slipping.
I think if you prime/bleed the air like it says after do do the change you should be fine.