I can't answer all your questions, and will ask a few in turn.

1. Did you bleed your clutch when you filled it up? It's just like replacing brake fluid and having to bleed those to ensure no air in the lines. It shouldn't cause "slipping" per say but I would put money that it would accelerate any problems you are experiencing.

2. You haven't changed your transmission fluid for 20k miles? What fluid did they use? I generally like to change my transmission fluid every 10-15k (Some people will do it more often) and with high quality fluid. (Search for what people like, I personally recommend the Scott Grey mix)

3. How savvy are you with tearing apart cars? I have never personally changed a clutch but there are a number of people that have on here and posted either How-To's in the garage or somewhere in the performance section. Also, make sure you think about if you every plan on doing more performance mods to your car that would increase the power significantly. If you are not, don't get some crazy stage-8439 clutch capable of holding a bazillion hp and ft lbs.

4. This totally depends on driving style and a myriad of other things. MY stock clutch was still doing fine at 162k miles without really any slipping. Some people burn through clutches in under 50k miles.

 

Welcome. Looks like you did your research before posting here. Sounds like you have a fluid leak and your system could have air in it. The leak could be from the reservoir/clutch master cylinder, at the other end of the clutch hydraulic line at the clutch slave cylinder or inside the car behind the clutch pedal. About the fluid being dirty, you should drain it, flush the system, re-fill it and bleed it again. That could cause clutch slipping. That dirty fluid could also be affecting your master and slave cylinder and if you want to go all out you could replace both. I don't think it's your clutch or your transmission. Look for that leak first.

 

This might be way out there, but i was told once that a failing CV joint can mimic a slipping clutch. :twocent: More than likely not, but had to put that out there.

 

120k on stock clutc is a good life, I changed mine at 110k and it was still good, but most likely you have a failing clutch. There is a great walk through in the Garage section about a DIY clutch job, If you plan to go this route I definately suggest getting poly moto mount inserts since they are going to be off anyways (there is a how-to for poly inserts) I suggest bleeding it just because. You need 2 people to do this one person pushes the clutch in other person opens the bleeder valve on the slave cylander, watch for air bubbles and then close it up and do it again. but the clutch pedel has to be pushed in until the valve is closed so you don't introduce more air into the system.