i use castrol power 1 semi-syn 10W-40 in my bikes...no issues with any of them at all.
you should be doing oil and filter changes every 3000 miles to keep your bike in good condition.
if you do suspect the clutch, ross46 did a cracking guide (
viewtopic.php?f=118&t=14839#p108533) with a good video, followed this exactly when i did mine and it was dead easy.
worst case scenario you need a brand new clutch assembly (6x friction, 5x steel, 4x springs and maybe 1x cable) which will run you to ~£120 with a forum discount from cradley kawasaki. my plates were fine but i needed new springs, and changing the clutch cable made it silky smooth
![Very Happy :D](./images/smilies/icon_biggrin.gif)
UK ZXR400 L3 (1993) - Fully restored and on the roads, my green beast!
JPN ZXR250 A2 (1990) - Revs to 19,200rpm... 'nuff said
![:smt003](./images/smilies/003.gif)