About a month now.... Changed it twice because of a WP seal failure.
Reason being is Gas Gas USA recommend 5/30 but i didnt have easy access to that so had to go with the 10/40, and they did mention Castrol and non synthetic, so kinda figured 'its castrol, its non synthetic (ok the weight is slightly wrong) but it hasn't had any adverse affects at all. The clutch feels nice, all the gear selections feel nice and positive, not too much drag, easy to find neutral etc....
Do you think thats bad??
I mean the only real difference is the 10/40 is a slightly heaver viscosity whether it be hot or cold...