I used 10-40 GN4... I used that on my 400ex and the quad lasted forever and took the worst beatings known to man. The previous two owners were friends of mine and weren't too nice to it. Synthetic is always good too, definitely lubricates better than conventional!
Just a pointer on your first oil change...be sure to let the quad idle for a few minutes before removing both drain plugs and the oil filler cap...this will allow all the oil to drain from the engine...I'm sure everyone here already knew this, plus it states it in the manual...but just a heads up!