Well...i used last season GRO 5W60 full synth but i think GRO discontinued it or my gasgas guy didn't take it in anymore...so this season i'll be using GRO 10W50 full synth.
It all depends what kinda riding are you doing. If fast tracks and no issue with cooling, i would say 10W40 is good. But if you do slow tracks, much clutch slipping then i would put 10W50 in. I'm changing my oil every 5 to 10 hours so will let you know how the 10W50 works.
But all that's just my 2 cents
