not sure where you got that info but its wrong. the clutch slippage has nothing to do with synthetic vs non-synthetic. it has to do with energy conserving. if you want to use car oil just make sure it isn't one that says "energy conserving" on it. Specifically you're looking to avoid molybendum (I am thinking of the right chemical right? I know it starts with moly)
castrol R4 5W-40. worth every penny for my investment. cheap insurance in my opinion. get it on sale for $6.50 a liter, otherwise it's $10. actually makes the bike shift better, and have noticed no slipage whatsoever.
A good synthetic makes a difference in my experience. Quieter operation and slight improvement in gas mileage.
I've used Honda HP4, Golden Spectro 4, Castrol GPS and am just now switching over to Shell Rotella 5-40. Available at Wal-Mart for less than $15 per gallon, from what I've read it appears to be more resistant to acidification than most other oils and is safe for wet clutch use.
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