what oil are you using?

[forchetto]

I've been using a reasonable quality motor oil 10w40 for years on all my bikes without the least hint of trouble. The last few years I've treated them to semi-synthetic, sometimes referred to as "synthetic base" but I draw the line at fully synthetic on Chinese engines. It appears that clutches may slip, the same applies to molybdenum disulphide additives, these really bugger up your clutch plates.

Sometimes there are explicit warnings about using synthetic, like on this site selling Zongshen engines:

http://www.monkeybike.co.uk/acatalog...ry_Clutch.html