What oil to use in a Honda ATV?

[drive it forever]

My brother has a honda atv sportrax 250 ex. I might be buying it off of him, he wants a 4x4 atv. Its air cooled, has the typical shared engine/tranny oil set up and has an auto clutch. He uses 10w40 castrol gtx in it. He is the type that wont read anything, just blurts out, I was told not to use synthetic, it will mess the clutch up. With no owners manual around to prove anything to him, does anyone know the oil honda specs for them?

 

[04SpecV]

They probably spec a Honda 10W30.
Anything that isn't energy conserving should work as long as you change it fairly often. Synthetic oil isn't what makes the clutches slip thats just a rumor someone came up with.
I use Mobil1 Redcap 15W50 in my Raptor. When I had the Yamaha 10W30 in it, it was really hard to shift the gears if you got the engine hot at low speeds.
Since the Redcap 15W50 is no longer avail, I will probably try Rotella 5W40 next.