What oil??

[Billi]

I know it's the age old question but with being new to this off road lark i'm still finding my feet so to speak.
I'm riding a 2005 Yamaha WR250f and would very much like to do an oil change and service on it. The book says I should use yamalube (obvoiusly) but a 20W40 viscosity. Now this seems to be pretty easy to get hold of if you live in USA but over here not so much.
I've spoken to a few folks and they say that 10w40 is fine to use.

Is this the case? Fully synth or mineral?

Why? Why not?

I want to ge this right so that the engine gets the best chance at surviving ownership with me as possible.

Many thanks in advance.

[Colin Blackburn (cee-b)]

I always ran my Yam WR250F on Castrol Power1 Racing 10W40. I did it on the advice of some pretty knowledgeable Enduro racers and never had any reason to question the choice.

[Billi]

Thanks for that, will take a look.