to find out if its the clutch, i would take it to a dealer (you can check yourself, but if your not mechanically inclinded i wouldnt)
how long has your shortshifter been installed?
Fuck taking it to a dealer to get it inspected.
Put the car in first at a dead stop and put the e-brake all the way up.
Rev the engine up to about 3k while slipping the clutch intermitently (sp), and see if your car jerks at higher RPM's, or if you burn the clutch easily.
Doesn't have anything to do with the oil, your clutch is just really really bad. I posted a pic of my clutch when I took it out, and it wouldn't hold above 6K RPM's, and it looked toasted. I can't even imagine what yours looks like.
Also, until you get it fixed, don't accelerate hard or burn the clutch. Once you burn it once, you will have close to zero traction on the plate, and the clutch will not be able to hold at any RPM's until it cools down.
Happened to me and I almost shit my pants. Burned it once while driving and the clutch wouldn't engage at any RPM. Had to pull over and let the tranny cool down for a good half hour. Good thing I pulled into the parking lot of a restaurant.