In the past I used Silkolene and it was excellent. However, nobody seems to sell it near me and I have grown bored of trying to find it so I now use the generic semi-synthetic 10/40 from the acricultural merchants. It comes in a blue container with a white label and I think it's for quad-bikes etc. It seems to work a treat and is very cheap, so consequently regular changes are affordable.
No matter which oils I have used in the past the clutch has always slipped a bit if pushed really hard. I think, basically, the clutch is a bit rubbish and not really up to the job. I keep meaning to fit stronger springs but I haven't yet found the time.