If it's your first clutch then you probably got some abuse on it earlier on when you were first learning, so it will wear out faster. I bought mine with 28000 miles of gentle use, and have since abused the crap out of the car with 55000 miles of driving plus auto-x. The clutch slipped when I was pushing 195whp at WOT, but that was 3.5 years ago. Clutch has been fine since and I am at 84000 miles now.Since I posted this I installed a clutch on my other vehicle, and I am going to assume that my chatter issue is indeed from the throwout bearing.
My gear shift sticks a bit too, but not always. It gets worse in the cold weather. I grind from time to time, usually 2nd gear. I'm using Pennzoil Synchromesh which I bought from AutoZone/Advance Auto. The bottle looks like
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I was using a RedLine synthetic mix (2 quarts MT90 and the rest MTL), and it worked well, but was expensive and I wanted to try something else. It's not fair to say which works better because things are wearing down now, so they won't work as well as they did at 30000 miles as at 80000 miles. I'll switch back to RedLine this summer and let everyone know my thoughts.
Smitty's MSP, the best bet right now is change the fluid and see how it works.
yudkib, snow tires are not a good thing to have to judge if your clutch is slipping. Those tires break loose so easily, and if it's slick at all out they will spin. Wait until spring with some decent tires and try. Get onto a highway onramp, shift into 2nd and gun it. If you get high RPMs without the speed, then your clutch is toast.