Hello there,
Probably far too late to reply to this one but, as I have been out of touch for ages, here goes.
After spending a fortune on Filtrate and Morris oils, I decided to chance a good quality, but much cheaper, semi synthetic oil. Works fine with no clutch slip at all. I don't thrash the bike at all, but do tend to "short shift", as the Americans call it, which would, I suspect, put pressure on the clutch.
This bike is a low mileage one, for me, at about 85,000 so don't know how long I will have no problems.
Hopefully as long as the bike lasts (at least another 50 or 60,000 on previous form).
Cheers,
Richard Black.