Which Mobil1 did you use? and what did your clutch plates look like?
What DSW was getting at about oils is that most automotive synthetic oils add friction modifiers. You can't use these with a wet clutch system, your new clutch won't last more than a few hours. Most motorcycle synthetics will advertise on the bottle "does not contain friction modifiers" or also know as "moly" (molybdenum, comes from the mineral molybdenite). So be careful when using automotive oil, read the bottle for contents or you could end up with some nice mirror-finish clutch plates.
With all that said, HD springs help, but go with a DP clutch or an EBC kit, they're nice for peace of mind.